I'll start out by saying that I've already picked out my bridesmaids and MOH. It was a no-brainer, seeing as my MOH is my sister and my other two bridesmaids are sister who I've known since they were born. So, they are sisters to me. They've always been there for me and we've all stayed close friends throughout life.
FI told me last night that his sister asked his mom if she knew who was in the wedding party and then why she wasn't in the wedding party. FI thinks this was brought up by his aunt, because his sister probably wouldn't have thought of this otherwise. His mom asked her if she wanted her to ask FI to ask me, and she said sure.
FI did a good job explaining that we already have our wedding party chosen, and that my bridesmaids are girls I've been close to all my life. I told him last night that it'd be weird to just include FSIL just because she'll be my SIL. I mean no offense to her, I just know I don't want her in my WP (she has very different views than FI and I when it comes to religion, and the result of that is something FI and I don't like and he's not on the best of terms with her).
Just an interesting story I wanted to tell. Hopefully it'll help some people realize that you don't need to include family (or future family) if you're not close to them or don't want them in your WP. Just explain politely that you've already chosen your WP (or haven't chosen yet) and move on. Don't let family pressure you into doing something you'll regret later (I've seen tons of stories where that's happened).