Why & how can I stop being so petty about this?
first off, if your bf was invited to his sister's rehersal dinner, you could have gone because i'm sure he could do a plus one. And why do you have to hold some grudge? Does it make you feel happy and content? Maybe just live RIGHT NOW and stop living in the past. who cares what your fiance's sister thinks? Really. Just be the bigger person, regardless. AND PS... don't sit around focusing on any of his sister's shenanigans - it's not worth your time or energy. I'm sure you have your own life to deal with.
What's your question? When she got married you weren't a relation to her, you weren't her friend, you were her brother's girlfriend. Why were you expecting a red carpet? Now it's your wedding. She wants to be a part of her brother's day. Let her if you want to. Don't if you don't. Bear in mind adults are supposed to get married not petty immature children.
You do it because it's important to your fiance. You don't have to let her know your secrets but you can be the bigger person. It's up to you to shut them down and let them know (nicely) that you weren't asking for advice. And in return for being the bigger person you tell your fiance that when you and he have issues, he cannot go and vent to his family. Let him go to his friends, let him go to his co-workers but anyone that he complains to about you - and he will - will hold every single thing he says about you against you because he is going to them in anger. It may not be how he really feels at all but he must understand that he should not air your issues to his family. Don't include them in the planning, just invite them to the wedding. Be kind. Be forgiving and let it go. It hurts no one but you and your husband. If you are marrying him, you are marrying into family dinners and holidays and these people will be the grandparents, aunts and uncles of your children. Make nice. You're going to want babysitters that work for free.
I don't get your title question because I'm not seeing anything you're doing that's petty. I do think you may need to have a chat with your fiance about boundaries. You don't want to drive him batty badmouthing his sister, but if he's telling her details about the wedding and she's using these details to harrass you, he needs to stop with that. In a comment, you said something kind of alarming about him asking "how high" when they say to jump. Make sure this stuff gets sorted out before the wedding. There's a fine line between being close to family (good thing) and not having a backbone to protect your wife from their meddling or worse (bad thing).
Try and forgive (never forget), don't let her steal your joy... That being said, keep your friends close and your enemies closer.
So what's your question here? If you want to let her in on the plans, then tell her what you have planned. If you don't feel like keeping her in the loop, then respond with something like "We haven't planned that part yet" or "We want it to be a surprise for everyone!" or "I don't know yet" and then change the subject or walk away. Or just ignore the question entirely. Remain polite, but just deal with her when you absolutely have to. Ignore/avoid her otherwise. You can draw effective boundaries with a relative without having to be nasty about it. Make sure your fiance is on board with these boundaries ... this won't work if you tell your FSIL that you want XYZ to be a surprise, and then she asks your fiance and he spills every detail. He has to be on YOUR side ... and, again, he needs to realize that it's OK to draw boundaries with a relative and it's not a mean or nasty thing as long as you're polite yet firm/blunt about it. "now that we're engaged, he expects me to forget everything she's done to me bc she's going to be my SIL" This is your main problem here ... why are you marrying a guy who's allowed his sister to be mean to you all this time, and on top of that he now expects you guys to be best friends? Not fair. You need to sit down with him and explain that, while you recognize that he wants his family in your lives, he also cannot stand quietly by if they are being nasty to you. Again, talk about setting firm boundaries with them. Maybe see a pre-marriage counselor if need be. DO NOT marry him if he cannot/will not set these boundaries.
Put away your years of resentment and thank her for her interest in your wedding planning. Tell her you've got things under control but will reach out to her if you need her help. As you chosen to marry her brother you can't really be in some tit for tat vendetta situation with her for the rest of your lives. Always remember when you marry you marry the whole family. You've decided to put yourself into the position where she'll be a constant. So find a way to deal with it or don't marry her brother.