What am I missing?

She is asking you for ideas, not for you to do it or pay for it.
Calm down.

@filmfann No, she asked what I wanted to serve. Not what I thought she should serve but what I want to serve.

Sorry @filmfann , but I think @WillWorkForChocolate needs to bring up the subject as soon as possible with the friend, and clear the air. There needs to be a definitive statement that you are not throwing the shower, and that you will most likely attend if nothing else is going on. And tell her you haven’t been planning to serve anything.

Then you can be gracious and offer if you feel like it.

You need to have an honest and clear talk with your SILBF. Tell her that she seems to have a misunderstanding. You are not throwing the party. It will not be at your house. You want nothing to do with it. You have other priorities. Being a forthright person, I think you already know this.

Oh, I know too well how “honest and clear talks” go with some women.

You need to give her a horse laugh, to her face. That’s honest and clear enough for a manipulative bitch like that.

Perhaps she thinks you are going to help her (which might be a reasonable assumption, I’m not sure). If she thinks it’s a team effort, that’s why she is saying “we.” If you’re not planning to help at all, just clarify with her that she should not expect your help.

Perhaps she thinks you are more experienced in these matters and would be able to come up with great ideas. I’d be flattered if someone requested my help and were interested in my opinion and ideas. As @filmfann says she is not asking you to do it or pay for it.
That said, if you don’t have the time or the will to help out for whatever reason, just gracefully let her know that right now you have your hands full and can’t really put your mind into organizing the event.

I don’t want to flat out tell her I’m not throwing a shower.

Why not? You have complained about family dynamics that revolved around your sister-in-law before.

You are very forthright on this site. Use the same techniques with the real-life-situation.

“Unless I am misreading what you are saying, there seems to be some confusion here. Let me clarify. I’m not throwing the shower, you are. I thought that’s what you suggested. I unfortunately don’t have the capacity to do much more than attend”.

That’s what I would say.

She may be awkwardly asking for suggestions for a menu your SIL might like and when would be a good time for the party. Some phrases that might come in handy: “I’m sure SIL will be fine with whatever you decide.” Or “SIL likes Mexican, Italian, American, and German food best.” “I have no idea what SIL’s schedule is like. You might ask MIL.”

Next time she asks you something I would just say “why are you asking me, I’m not the one throwing this party!”

It seems to me like she is trying to be inclusive of you and not necessarily looking to you to do all the work, therefore, in order to keep a good relationship going forward, I would just tell her in a casual, off-hand way that it’s her party and you don’t want to step on any toes, but if she’s “really, really stuck” to let you know (if that’s how you feel). I am assuming you would not be totally adverse to helping if she got desperate.

Perhaps take a shower?

At least she isn’t asking you to organise a golden shower, but I agree you need to talk about it.

@TheProfoundPorcupine you stalking me too?

@Shippy did the sound of the large plant pot being dragged along give the game away? I needed something to hide behind.

Clarification, or so it seems.

Thanks, everyone. Yes, I plan to make it perfectly clear that I won’t be providing food and decorations for the shower, the next time I talk to her.