Should I tell my adult sister that my 9 year old doesn't like her fiance?

No. Keep the peace between them and between you sisters. They are moving away so you won’t see them that often. This might give your daughter time to get over her dislike of this guy or not. Either way it’s not worth rocking the boat.

You could say to your sister that seeing relatives brings up memories of grandma and makes her feel sad.

What would your motivation be in telling your sister? She can’t do anything about it because it has nothing to do with the guy himself. It has to do with your daughter’s perception of the guy taking gramma’s place somehow. I’d want to dig a little deeper to find out she feels that way. It would make more sense if your stepfather remarried. Then you’d have someone who really would be “replacing” Gramma.
What connection does she make with her gramma and your sister?

Why not talk about it with your daughter and your sister at the same time? Have her over before she moves, let your daughter know she’s coming and you’d like them to talk about it.

You know your daughter, but that’s what I’d try to do. Making her explain it will clarify it for your daughter as well.

I’m thinking when we went out on Christmas eve, or had the gathering on Christmas day, instead of “mommy, daughter, sister, stepfather and Nana” it was “mommy, daughter, sister, stepfather and boyfriend.”

Well, she just needs maturing. I’d talk to her, and let her know she can talk to you, but don’t make a big deal or it will impress itself more firmly into her brain. She just needs to spend more time with him and them. He’ll slip into his proper place in her mind, naturally.
No. I wouldn’t say a word to anyone else. Just let it ride.

Sounds to me that your daughter is still grieving over the loss of her Gmom. Death of a loved one for youngins can be a confusing and traumatic experience. Tell her how no one will ever be able to take your mom’s place and how it is important to appreciate the people in her life. You could probably tell your sister this as well and let her know your daughter is still adjusting to the loss of her Gmom.

That’s a good thought too. Tell your sister just so she’ll be aware of how the child feels and why.

No, just leave it. No need to stretch things further. No need to hurt your sister’s feelings. Your little daughter will grow up and mature and see the whole thing from a different perspective. It will all fall into place.

I also would not tell your sister. Let your daughter talk out her feelings with you all she wants and validate them. But why tell your sister since she is going to be with this guy? It will only make things awkward for her and your daughter. You can definitely tell her that she is still grieving your Mom and seeing family members triggers her memories but I would let it go at that.

Exactly what @janbb said.

I think your daughter is naturally upset that her Nana isn’t there anymore. I think this is where her apparent dislike of your sister’s fiancé comes from. Your sister knows something is wrong and I would just put it down to your daughter coming to terms with the death. Your sister likes her fiancé, you think he’s a great guy. I expect your daughter will like him too in time.

In the meantime slip some memories of your mother into conversations with your daughter. Let her talk about her Nana if she wants. Those memories are something you share and you can help each other come to terms with the loss.

@janbb has it. There is no benefit to telling your sister.

Help your daughter work through her feelings about it all. Do let your daughter know that her behaviour / attitude has been noted. She is old enough to be able to modulate that when she is not being monitored by you.

I was your daughter when I was about 4. My mother had to speak to me about it – I was caught trying to discreetly kick my uncle whenever I thought no one was looking. Some jealousy/mourning stuff going on. I learned that I couldn’t take my feelings out on him. He eventually went on to become one of my very best chums.

Since they are moving away probably don’t say anything.

However, if you feel it’s better to say something, I wouldn’t say your daughter doesn’t like him, I’d say your daughter feels like he is there and grandma isn’t, and to her it’s sad, because she sees it like he is taking grandma’s place. It’s just how she is sorting her realization that grandma is no longer along when the family gets together, but someone else is. I think it will easily be chalked up to a 9 year olds way of framing things and not be offensive, and it will be understood it will just take some time.

I wouldn’t mention it to sis, either.

Other than that, select the advice above that most nearly suits your relationship with your daughter and sister.

I thought about this more. If you were my sister you definitely could say to me your daughter is having a tough time with my fiancé, and the reasons why. If my SIL was your sister I would tell you to say nothing. So, it depends on what type of person your sister is, and close you too are, and how much you generally tell her about your daughter and how she is doing. If you talk to your sister all the time and are accustomed to sharing everything, it might feel odd to keep it to yourself, and have her wondering what’s going on.

In thinking about it further, if my daughter doesn’t know someone or doesn’t like them, she doesn’t usually talk to them much at all. At the diner we often go to, the waitress who is our “usual” is friendly with my daughter and vice versa, but another waitress will barely get a response out of her. To the point where another waitress asked our usual waitress “Is she OK?” in reference to my daughter (our usual waitress told me later). My daughter takes a while to open up to people but she did tell me specifically she doesn’t like my sis’s boyfriend.

I am still not sure what to do. I don’t want to cause a rift between me and my sister. I did tell my daughter this morning that even if you don’t like someone like this person, we try to be nice to him because he’s going to be in our family and we’re going to see him a lot.

The thing is, it’s not that she doesn’t like him. That’s not it at all. It’s that his presence causes her sadness because of the association with Gramma. I’d definitely not use the words, “She doesn’t like him.”
If there is a reason for it to come up, like if your sister asks why she’s acting that way (which she already has,) I’d explain what’s going on in her developing mind about the association.
I’d also try to steer my daughter away from the “like” or “dislike” aspect to give her a clearer understanding of why she’s ambivalent toward him. She’s going to need some help working this out.
You know the situation better than us, but another thought is that if you tell your sis and her fiance what the deal is, perhaps they could come up with some ideas to help her work through it too.

Or, maybe she really doesn’t like some aspect of his personality. Have you asked her for specifics?

Up front, I’m still very much trying to figure out how to deal with this sort of situation with my daughter, who is an opinionated and feisty little young lady. So I’m not trying to speak from anything other than understanding how hard it is to deal with the rest of the world and help little people come into their own.

My rule of thumb (so I can remember it in the moment) is “acknowledge, don’t accommodate” for behavior I don’t want to encourage. In other words, “hey, I see you are angry/upset/sad, but pouting isn’t the way we should handle this situation, what else can we do to make this right?”.

I let her know her opinion is valid, but that handling it in a negative way isn’t ok. She learns how to handle it better and that I care.

To me, she’s going to learn something in that moment either way, it’s going to reinforce something whether it’s positive or negative, so why not at least start building skills to deal with it when she can comprehend what she’s feeling and what’s going on.

If your daughter was a teenager, she’d be expected to at least be civil to your sister and her fiance when your family gets together. She’s not that far away. My thinking with my earlier answer is that if she’s at all close to your sister, then having them talk about how happy this new man (to your daughter) makes her, and how important he is to her might be all that’s needed. That’s all the adults in your family need to know, so at some point she’ll need to learn that is part of being a family member.

Again, you know her, and all the people involved better than myself or anyone here. You know what they all can handle, but I don’t think setting the standard that it’s ok to isolate and be unwelcoming to someone new is going to serve anyone well in the long run. Why not teach her how to handle it better while it’s still totally expected that she’ll need it explained to her?

You’re sure he hasn’t done anything specific to make her uncomfortable, right? Like my dad used to have a friend we didn’t like, and we always liked other adults who came to visit. We would want to stay in our room and would ask our dad not to have him over. The reason was because he got too close. He once lifted my shirt and touched my stomach commenting on how skinny I was, but it was weird. My sister felt uncomfortable around him too. I think he said other uncomfortable things to me, but my memory is very vague about that, I just remember not wanting him to come over.