Just let them know that you are there for them and that if they need anything at all, they should ask. Just be as open as possible with them. At the same time, do make sure mom gets rest and isn't doing too much...offer to help when you see the opportunity, just don't do things without asking and you'll be fine.

my situtation was a little different but hopefully this may help you?
when i was 14 my girlfriend had my son and my mother adopted him and raises him as though he's my brother.
it hurts a lot to have someone raise your child BUT no way could i have done it on my own, and at times when my mum and step dad have gone on holiday and i've babysat my boy its been horribley hard and i've just counted down the days when they'll be back so i can rest, or have a bit of fun.

So the moral of that i think is that they'll probably be a bit annoyed with you helping out too much, but at the same time they'll be dying for a babysitter and someone to help! young babies don't really sleep enough and they'll love it when grandma cleans the baby and plays with it so they can have a nap

Thank you both for the great advice.