Opinions on having fsil in bridal party?

My fiance and I have been engaged since March. I had talked with him about having his sister in the wedding soon after and felt that if he wanted her in the wedding I would ask. I had only met her twice (she lives in Alaska) and isn't home often. We had decided to have his nephew be the ring bearer and decided his niece would still be too young to be the flower girl so I asked my cousin (i have no nieces). I asked my two sisters to be my maids of honor and also asked a long time friend and another close friend from here. I figured four was enough. He's a slacker and finally decided on asking his brother in law to be an usher. His mother has made several comments about how they had her family in their wedding party and his father also made similar comments. She also made comments that my sil has never been in a wedding. i mentioned how one friend may actually not show up and she said well you could always ask fsil. I'm feeling really guilty that I didn't ask. Should I ask her now?