Babysit or no?

SIL is asking me to babysit her almost 5 year old and 20 month old tomorrow from 12-3:30. I have an 18 month of my own and just gave birth to my second 6 weeks ago. Her 20 month old and my 18 month old both have a nap time at 12. Nap time is already hard for me because most of the time I have to leave my newborn to cry while I put my toddler down.

I want to be a good SIL and aunt but jeez, 4 kids is a lot especially since I just had a baby not too long ago. What should I do?