Bridesmaid (sister in Law) is ignoring me 😞

I am in a pickle. I have a very small bridal party (My MOH, my brothers girlfriend, and my fiancé’s sister). While my MOH and the my brother’s girlfriend have been very involved in all the process (dress shopping, color picking, venues etc.) I haven’t gotten my fiancé’s sister to act remotely interested in anything. She is always invited but ignores me completely until my fiancé has to get a hold of her to message me back. In person she’s very nice but we honestly don’t know each other. I thought it was etiquette to have her in my bridal party but now I can’t help but have hurt feelings. She hasn’t gotten back to me at all in regards to her dress and I’m really worried about her getting it in time for alterations before the big day. I am not someone for drama at all, hence why I’m scared to say anything, and I understand she is a very busy single mom working odd hours. I asked her a few weeks ago if she for sure wanted to be a part of the bridal party, and she said she did. I don’t know if she said that just for the sake of keeping peace though. At this point when our deadline for dresses is coming up I have to either trust her or face the consequences of asking for her to be a guest rather than in the bridal party. Her daughter is going to be our flower girl and she’ll need help going down the isle due to her age. I might ask her to do that instead, but at the same time I’m afraid of starting drama with my fiancé’s family. Advice pretty please, not criticism.