So, my mom brought up an important issue with our wedding..
My fiance and I decided we want a small wedding, including a small bridal party. We decided we would both have 2 people stand up on each of our sides. On mine: my sister and my best friend. On his: his two closest friends. Neither of us have brothers, so there is no issue there.
However, he has three sisters.
My mom asked me how I plan to incorporate them in the wedding! I need ideas!
How can I have them play roles in the wedding so they feel involved?
I'm open to all of your suggestions and look forward to your feedback!
Thanks in advance

Re: His Sisters?
If you and your FI want to include his sisters, and everything that entails, you could ask them to do readings. If one or all are musically talented, you could ask them to perform during the ceremony. Depending on your ceremony, they might take another role, such as being involved in the presentation of the Eucharistic gifts if you are having a Catholic mass.
Beyond that, there are not many roles that you could offer them that aren't considered work. You might ask them to be ushers/greeters - but that isn't universally seen as a "good" job. For example, it's an honor to be a greeter or usher where I grew up, but seen as a second rate job among my friends.
If you don't want to include them in any of those ways, you could always consider having them be VIP guests - get them corsages or boutonnieres, seat them in the first or second row, include them in a few special photos - them with you, them with FI, all of you together. They can still be included in showers or bach parties, and you could invite them to get ready with you (maybe invite them to join in some of the pre-wedding pampering too) if you wanted.
ETA: paragraphs
First off--are you including them because you and your FI want to (mostly, because it's important to FI?). My FI has a brother and a sister, and we are not incorporating them because in his family, this is not a big deal. If it's important to your FI though, it's worth exploring roles for his sisters, but I strongly object to being forced to have ANYONE in your wedding party that you don't want. You can do things like invite them to get ready with you, or get their hair and makeup done with you and your bridesmaids. Even little things can go a long way to make them feel included.
I am going through the same thing, except with my 2 brothers. Ideally, they'd stand up on either my, or my FI side since we're close, but that won't actually work out. My mom was at first VERY upset by this, but told me as long as I incorporated my brothers, she'll lay off.
To incorporate them, I'm doing the following:
One of my husband's sisters was not in our wedding (she was SUPER pregnant at the time) so we got her a corsage and made sure she was seated with his parents. You could have corsages for each of them, and then have the ushers seat them when the ceremony begins.