Personally, I don’t think it’s rude, but then again, like other posters have said, it’s pretty dependent on family dynamics. Do you have any brothers that your Fiance isn’t asking?? That was 1 of our issues.. we’re having a big wedding party (7 on each side), and I have 3 brothers, and he has 1 sister. I was originally including his sister, even though we’re not that close, from the start. When I told my parents who my bridesmaids were, they asked if my Fiance was asking any of my brothers to be groomsmen. I said no, because at the time, he wasn’t, and they seemed a little surprised & my dad seemed maybe even a little hurt, but let it go & didn’t make a big deal about it at all.
My Fiance kept asking me, “Are you SURE you want to have 7 BMs? That’s a lot of money spent on gifts etc…” and I told him I felt that I couldn’t cut anyone out of my group. I had my only sister, his only sister, my best friend from high school, my 2 college roommates who I’m really close to, and my 2 best friends in grad school right now, who are actually here with me to be able to help with stuff if I need it. I thought about it, and thought if he wanted to cut it down, and he wasn’t asking any of my brothers, I could just ask his sister to do a reading at the Mass instead of being a Bridesmaid or Best Man, especially since she’s not the girly type to be crafty or really be into any of the DIY stuff I’m planning to do.
I asked Fiance if he thought his sister would be offended if i asked her to do a reading instead..he was kind of taken aback that all of a sudden I might be excluding his sister from the wedding party, but said she probably wouldnt care, but I might want to ask his mom for her opinion since she knows his sister better. I have a very good relationship with his parents, so I went ahead & asked & his mom seemed very hurt at the idea that I wouldn’t ask his sister to be a Bridesmaid or Best Man.. “He has but one sister, and of course we’d love to see both kids in the wedding party.” To be honest, it kind of irritated me that she said that, because I was like well, my parents are paying for this, and they’d love to see their other kids in the wedding, too, but my FI’s not asking any of them…
Eventually I talked to him, & said I thought it’d be a nice gesture if he included 1 of my brothers, since I was including his sister. I have 3 brothers, but he only asked the 1 he was closest to, which I think is totally appropriate. And we even have a big wedding party!
I think if you’re only having 4 & want to keep it small, and you already have 2 sisters and he has 3, you already can’t include all the sisters. I think you could definitely ask the other sister to do a reading at the ceremony & have the youngest one be in charge of having guests sign the guestbook. My mom’s cousin got married when I was about that age, 13 I think, and I’m pretty close to him, so he wanted to include me in the wedding somehow.. I was WAY excited at 13 at the prospect of getting to do the guestbook & be a part of the wedding. If you don’t know his other 2 sisters that well, I don’t think you should feel obligated to ask them!