Two part post here....

Part One to adress the soon to be sister in law issue- If your soon to be husband asked you to have her in the wedding, if it means that much to the man you love I dont understand the issue. If you can only have two ask him if you can have three if you include her. It will make her feel good, make him feel loved, and make you look good to everyone.

Part Two to address the child with adhd issue - I agree signing the guest book would probably not hold her interest. I would discuss with your fiancee what the issue is with her being a jr bridesmaid....this may open some suprising dialogue on his views on children with and/or without disabilities.

Winter