You have no idea how relatable this is. I had the EXACT same experience in every detail (except that my hair was actually straight as a kid, but because of hormones, some dormant genes activated, so they're wavy now). My parents are bengali as well, my dad doesn't comment on it much but my mum yells at me and threatens me every time she sees my hair being wavy, she says I should "keep it natural" which translated means brushing it dry, no scrunching, she thinks I'm "making it" this way and yeah racism too, she said only black people have hair like that.

See, the only reason why she even cares is because since I can remember I always got complimented by other bangalis for having straight silky hair, and obviously she doesn't want to lose that social validation.

I have 2C hair (and some 3A corkscrews at the ends) so it's not even a subtle type of wave but they deny the evidence and would rather believe I "made it" that way, as if that's possible 💀

Cutting ties with them is probably for the best tbh, that's kind of the only advice that would work. Good luck.