My fiance only has one sister who is 5 years younger than him. Initially,she had some struggles with him spending more time with me vs her when we were first dating, but I feel like we have all totally gotten past this since (my fiance and I have been together for several years now). I love her very much and want her to be involved in the wedding. I have 2 sisters (no brothers) and since she has no sisters, she has always made a big deal about me being her 'sister'. I do get the sense that she wants more from our relationship some times, but I am a bit introverted and therefore not the best at cultivating the relationship (and she is a bit shy in some ways too).
Now that my fiance and I are moving forward with our relationship and getting married, I want to use this opportunity to express that she is important to us.
So our wedding party.... right now I am thinking my 3 close friends, my 2 sisters, and my fiance's sister. However, he has 4 groomsmen that he wants. I know uneven numbers in a wedding party is fine, but I like the symmetry of even 5/5, 6/6 (or even-ish 4/5) numbers (not 4/6).
SO we are considering asking if she wants to be on the grooms side.
However, I do not want to hurt her feelings or make her think I don't want her as a 'sister' or anything like that. She has also been historically a bit of a tomboy and has struggled with her weight and self-confidence, so I don't want her to feel uncomfortable from that point of view either. Plus she would be the only girl on the grooms side (and there wouldn't be men on the brides side either).