Sister in law bridesmaid question...

Particularly my fiancés sister.

I don't necessarily get along with her but we haven't had much of a chance to hang out. She makes me very nervous, but I can't not ask her to be a bridesmaid, one, because she's my fiancés sister and, two, she would make a huge fuss if I didn't ask. (I feel bad cause I already asked her daughter to be my flower girl and I'm pretty sure my fiancés asked her husband to be a groomsmen.) 

I know she loves tea and her house is filled with Paris related items and teacups. 

As terrified of her as I am I'm dying to get to know her better as she's supposedly "in love with me". 

Her birthday is coming up and I'm hoping I can find a cute way to ask her and hopefully jumpstart this inevitable lifelong relationship with my future sister in law.