Are you closer to one sil than another?

You're not obligated to hang out with SIL #2 just because you're about to be family. If you don't want to include her in everything then don't. She needs to understand that you and SIL #1 had a friendship for years so you're closer to her.

Take aside SIL 2 and explain to her that you and #1 were friends before becoming SIL, and continue to be friends because you enjoy spending time together. And while you likely will get to know #2 better over time, who you choose to spend time with is in general not her bisiness. Fairness does not come into it, as long as she is not left out of whole family gatherings. Most people spend more time with some siblings than others. Most people spend more time with some in- laws than others. But she needs to stop pushing being included when you and #1 have made plans on your own, because she is only being annoying and irritating both of you, not a path to friendship. And separately, say it to the relative she is married to.
Don't just let it go. Don't let her make a whole family issue of it.

View Quoted Comment I agree with this. There is no obligation.

Totally agree! You are not obligated to invite her just because you’re family.

View Quoted Comment Welp, I’m glad everyone here seems to agree that we don’t need to hang out with her lol. It’s just that she has a FIT. Her two brothers are closer in age and therefore they have the same group of friends, so we go out to the same events/parties. She tells on us to her parents...she’s 27. I just can’t deal with her. Here’s the thing: she and SIL#1 used to be close, until SIL#2 stole $ from the family and became a complete liar. So she wants to hang out with SIL#1 (not me, she doesn’t like me) but SIL#1 doesn’t want to for obvious reasons.