If my sister & husband need a date night and want me to watch their 2 kids, wrong for me to ask for $?

Sis and her husband never get "alone time", except at night when kids are asleep. They have a 7 month old & a 5 year old.

My sis desperately asked me to watch her kids tonight so her and her husband can get a break as they're both burnt out and want time alone together. Probably be gone 4 hours.

The 5 year old is independent but obviously the 7 month old is not. I don't have kids but from visiting sister I know the basics of caring for my niece. This will be the first time I've ever watched them.

Is it rude for me to ask for payment? Like $100?