How much should a 13 year old be paid for babysitting?

I'm 13, girl. I am babysitting my cousins tonight, but probably several more times throughout the summer. It's a 4 1/2 and 1 1/2 year olds, both boys. If it makes a difference, it's pretty much give them baths, put on pjs, go to bed, and a bottle for the baby. So it's not a lot and they only live like 25 minutes away. It'd be from approximately 7pm-12am ish. This is my first time babysitting ever, but I'm super good with them and such. So how much should I be being paid, if at all (since their family)? Thanks!