I’m curious why the focus is on the future SIL not including you in her side of the party, versus your brother? I know that historically there has been gender segregation in wedding parties, but this is a great example of why that really doesn’t make sense. Each part of the couple should feel free to include their special people, regardless of gender.
I got married last fall, and I made sure to include my siblings (only had one attendant, my sister who I’m extremely close to, and then had a functional role for my brother) and my spouse didn’t have any attendants, but I would see it as his role to include his family as he wanted.
I’m not really close to his family; they are friendly and nice to me, but there is also a lot of homophobia and I’m gay (which is easy-ish for people to ignore since I’m a bi cis woman married to a cis man, so….), so it’s just not something I choose to engage with all that much.