Christmas money - how much do you give your grown up kids?

I usually send our adult daughter money for Christmas as she's lived away from home for years and we rarely see her for Christmas (she loves to travel a lot).
My partner and I are both disabled and retired from work so we're not exactly rolling in it, so when my sister and I were discussing what to get our kids for Christmas she surprised me when she said she gives her son and daughter in law money for Christmas too so they can buy whatever they like, but she gives her daughter in law the same amount of money she used to give to just her son when he was single, so double the cost.
Our daughter has a steady boyfriend who she's been living with for a couple of years but I've never thought to double the amount I send her since he came along - I just presume he gets a present or money from his own family.
My sister thinks I'm being a scrooge not treating our daughters partner to the same as our daughter but we aren't flush and can't afford any extra expenses.
AIBU? Or is my sister doing the right thing with her family? It's something I'd never considered until she brought it up and now I feel like I'm being mean