Would it be weird to have 2 little sisters at the birth of my daughter?

Ask your fiance what she wants. The people you choose to be there should be people that she feels very comfortable with and who will be encouraging and positive during the birth. It doesn't matter if they are women who are mothers or not. Sometimes young women (or even men) who have never given birth are more sympathetic and kind than those who have given birth themselves ("I did it, you can too. It's not that bad, get over it."). It just depends on their personalities and how they get along with the laboring woman. Two young girls could be helpful if they are cheerful, positive, encouraging and mature. If they are ditsy and likely to great freaked out by blood and stuff, they shouldn't be there.