I have left DD with DH overnight but not with anyone else....and she is now 3 1/2.
To be honest while you are breastfeeding I wouldn't even entertain the idea.... but you need to be comfortable with leaving her and if your not then don't be pressured into it.

I would probally sit down with DH's folks and just say thank you so much for the offer but at the moment Im just working on breastfeeding and settling bubs into a good sleeping routine....

Maybe if you have room you can have them stay over and look after her for the night while you sleep!