Should I have asked my fiance's sister to be a bridesmaid?

I'm getting married in August, my fiance has a sister close in age, but I didn't add her to my bridal party. I didn't think about adding her until his mom hinted at it revently. But I already have 11 bridesmaids. Is this a big deal?
Should I have asked my fiance's sister to be a bridesmaid?
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