You are right that it is cultural, they’re south Asian although I’m not.
I paid rent for the 8 months I stayed there and worked, we now live in a private accommodation that my husband and I pay for.
They’ve treated me so horrendously when I lived there that I don’t know what is normal or cheeky behaviour anymore, and sometimes I wonder if my anger is justified or if it’s just influenced by past behaviour.
BIL would ask me to make his lunch, butter his toast, make him tea, all while telling me I didn’t have a seat at their table (figuratively), so when I see that behaviour replicated when he’s in my house, I really struggle to understand if it’s normal now that he’s a guest or if it’s just a continuity of the way he saw me back then.
My other sister in law is very much loved (other brother’s sister, not that brother), and they have all treated her very nicely, although she is from a completely different cultural background as well, BIL buys her stuffed toys, sweets, hangs out with her, is very kind to her in general.
They have explicitly told her they were only happy with her, that they loved her, and that they didn’t want my marriage to their son to ever happen, despite me doing as much as I could to be helpful.
MIL would constantly pressure me to move, and when I am about to move she’d beg my husband not to, that she needs the financial help (rent) and the domestic help around the house, and it would happen again and again, daily berating.