.When you're creating a bridal party, family factors can impact your decision about who to pick for your “team." But the short answer to whether or not you have to include a family member—or anyone—as a bridesmaid is “no." This is your wedding and you can do whatever you want—there are no rules. But while it's not technically rude to exclude someone, it can lead to hurt feelings. To do what's best for you (and your family), and ask yourself these questions: How important is to to you? If you're deciding between having your future SIL and another friend in the bridal party, how sad would you be to have to exclude your friend? Will it really matter on your wedding day, or ten or even twenty years from now?
How important is it to her? Your sister-in-law will be in your life forever, and if excluding her will hurt her feelings, is it really worth it? Chances are, even if you're not BFF yet, she will become more important each year from here on out. But if she could care less, it's probably fine to leave her out of the lineup. Go with your gut on this one.
Can you change the party number? You can always have your groom add another guy to his gang so you can add his sister—he'll probably be happy to do that in order to make his sis feel included.
Can you honor her in a different way? If you really feel like you're in a tight spot and can't budge your bridal party picks (it happens), give your future SIL a special place in the proceedings. Maybe it's lighting a candle with one of your own siblings during the ceremony. Or consider making her an usher.
Source: theknot.com