That was very nice of you to ask your SIL! I definitely think you should have who you want up there with you - not people just out of politeness. I only asked my 2 best friends to be co-mohs, and one isn't coming now because of pregnancy. Some people are telling me I should "replace" her to "fill in the numbers," but I really don't care much. I'm going to leave it to later to see if I want to ask someone else or not (I alrady have the dresses purchased)
Personally I've been a bridesmaid/moh 4 times now, and have dealt with so much drama on the other side. I think that's why I don't ask my bridesmaid much in terms of helping w/ the actual wedding. They had a Bach. for me and they're going to have a shower - other than that, I just want them to show up the day of the wedding. I haven't asked them to spend time helping me assemble oot bags or passports or anything because it'd honestly be not worth the hassle and I'd rather just do it myself then have to listen to complaints/moaning.