What would you tell this kid?

I would stay out of it.

Gently explain that soon she will be on her own and leaving the home.
While her father will still be at home rearing a baby.
Her life choices will be different than his and she has to realize that.
If in the meantime she wishes more time with her father then suggest a time suitable for both.

I would just be nice to her and make sure she knows that she’s loved.
That’s about all you can do.

We need a little more information?
What’s your age, sex and living arrangement?

For example, are you older than your sister, a guy, and living independent of both your parents?
How cool is your step-mom?

Is there a teen-aged pregnancy class you can get your sister into? Like where you carry a sack of flour, or a fake baby?
It might be a learning experience and make some fun for her.

Give us some more information and we’ll see if we can come up with something more.

I think the work has to be done by your Dad. He’s the one she needs reassurance from. If you have a good relationship with him maybe you can help facilitate this by talking to him about some of her concerns. Otherwise, the main thing you can do is listen to her feelings. The truth is that it will change things; that’s inevitable.

It would be easy for your dad to mishandle this delicate situation. His attention is going to be stretched in many directions, and in the struggle to deal with that it wouldn’t be surprising if he were to fumble the ball in some of his relationships. He may know that he will continue to cherish his older kids, and may be hoping/assuming that they will have enough confidence in the relationship to allow him to divert his attention for awhile to this new situation.

But that hope may very well be unrealistic. Divorce shakes the confidence that kids have in the permanence of emotional bonds, so it’s not at all surprising that your sister would feel insecure about this.

I would second @janbb ‘s suggestion. You may need to clue your dad into the fact that your sister needs reassurance, and that she really needs to be on his radar.

The key phrase in your question, which jumps out at me anyway, is ”[m]y dad is not good at this stuff”.

Before I proceed, I have to say that some of the advice above to “stay out of this” is sound. If your dad has been “not good at this stuff” for (presumably) your lifetime and that of your sister, he’s not likely to change right away. So it’s going to be an uphill battle to suggest to a teenaged girl to “fix” things between them, and depending on the amount of finesse that can be transmitted to you to transmit to her… the chances of doing good through specific advice that we give you vis-à-vis him-and-her are very apt to turn a livable though imperfect relationship into a living hell. So do be cautious about that kind of advice.

It’s also not at all clear how your sister gets along with her father’s new wife (stepmother?), and the dynamics between her mother (also your mother?) and her father also play a part. So I’m loathe to suggest specific interpersonal things between your sister and her Dad’s new wife, either.

What I can say with some assurance is that if your sister would like to improve her relationship with her father’s new wife (however good or bad it might be at this time) and the mother-to-be of her step-sibling, and even more especially if she can help her with the pregnancy during her weekend and other visits and most especially of all, if she can help to relieve the new mother from some of the work load that new mothers have (certainly, if it’s her first baby!) then she will be indispensible to them both. If she gets along with the baby, can be trusted to take good care of it and learns how to care for it (changing diapers, feeding and burping, keeping warm and safe and out of harm, putting to bed and calming it when it cries, for just a few examples), then she will never be forgotten, and will be included in all kinds of ways for the sake of her utility and helpfulness, if for no other reason. Get in good with Dad’s new wife, and have her sing sister’s praises to Dad, and Dad would have to be an absolute troglodyte not to notice and care, himself.

Thanks, all. I talked to my dad today, and it went okay. Very exhausting, because my dad is completely overwhelmed, and I am overwhelmed by how sorry I feel for him.

I hope I’ve helped him see that he is needed, at least, and maybe given him some ideas on how to approach the subject.

I like the idea of letting my sister know that while her role will change, she might even be able to (eventually) connect with dad through the baby.

PS: Because my family (used to be) active on Fluther, I don’t want to give more details.