Where does sil belong?

So my FH have been back and forth on the issue and it's the only thing that's caused major disagreements during the planning process. We can't agree on whose side to put his sister on during the ceremony. I know it really doesn't matter, because she will probably get ready with me and she is invited to my bacherlotte if she wants to go, which I doubt. On my side I have my two best friends, my sister, brother, and youngest sister as a jr bridesmaid. I'm pretty close and trust all these people. On his side is just 4 close friends, some he's known since he was a child.
When we first started planning, I asked if my brother would be on his side, and he would always get very quiet about it. He didn't say it, but that meant no. Since my brother is important to me, I decided to have him as a bridesman. I thought logically that FH's sister would go on his side and it would be no big deal and he wouldn't even care. Well, he does care about this. I'm not sure if he thinks she will throw off the balance in his bro group or if it's just the fact that she's a girl and he doesn't know that either gender can be on either side. He's pretty adamant that she will not be on his side.
How do I deal with this? It is the only thing we keep arguing about and can't come to a compromise on. Should I just suck it up and put her on my side? I know she wants to be up there because she sent me a text right after we announced our engagement and she said she wanted to be involved in the wedding. She obviously isn't going to be planning, so this was her way of asking to be in the wedding party. Honestly kinda rude to me, but whatever.
Somebody please shed some light on this situation!