Should I ask my fiancé’s sisters to be bridesmaids?

I’m incredibly in love with my fiancé. I adore his parents, though I absolutely hate most of his family, namely his sisters. They’re overall rude and really just plain weird and socially awkward. My fiancé is planning on having my younger brother as a groomsman. He understands that I do not want them in the wedding. I am also planning I’m on having my 6 year old sister as my flower girl. The problem stems from his mother. She’s repeatedly mentioned that it’d be nice to have his niece be one of my flower girls and her daughters to be part as two of my bridesmaids.... I love his mom and I don’t want to feel like I’m disappointing her. My fiancé says that she’ll understand and it’s not her wedding, but I really want opinions. Also She’s not pushing it on me rudely, so please don’t hate on her lol.

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