How much should I spend on my nieces/nephews?

I have 2 nieces and 2 nephews from my half sister, I always buy them all for birthdays and Christmas, my sister doesn't buy anything for me although she buys her brother/sister from her mothers side who are the same age as me, I know it's not a competition and I'd never bot buy for the kids, I don't buy my sister birthday/Christmas gifts as I buy the kids but I still buy her a card and wish her happy birthday even though she doesn't for me.

I am a disabled young woman therefore I can't go to work so if I was to give each child 10.00 for birthday/Christmas that would be 80.00 a year which I can't really afford so if I was to give them 5.00 that would be 40,06 but is 5.00 for each birthday/Christmas enough?