My sister just had her first and is grateful that my hubby and I are going to be staying with her while we move. BUT, we're very close and I told her very clearly what my expectations were and asked what exactly I could do to help. It's boiling down to me cooking and keeping the baby while she naps. She rejected offers from everyone else...and I think it's because she knows she can be blunt and honest with me and I won't be hurt if she needs to have a little time to be mad.
I think if you're going to "impose" make sure you ask and go to truly help out. (My sister had offers where what they really wanted to do was hold and play with the baby and expected her to do everything.) Maybe you could even let them know how you intend to help or ask how you can. Really set the boundaries and expectations. Emotions can run a little hot and cold after giving birth...for both parents.
I hope it all works out. If anything enjoy your time with the new baby and CONGRATS!!!