I agree with a lot of what has been said here about establishing limits. My husband and I have been together 18 years and it can still get frustrating. I am not good at establishing limits with my family of origin.
Some of what you are going through could be about how that side of the family lives. In my family of origin we all live close to each other or visit a lot. Not that many decades ago here and around the world families lived in the same house. Different generations and brothers, sisters, aunts, uncles, grandparents etc. I see a lot of this still happening with new immigrants. It's just the way it has always been for them. North America really doesn't live this way much any more. It wasn't until I read a book that talked about someone from North America marrying another person from another culture & they couldn't figure out why after they married they would still live in the family home. In North America we grow up, leave home and establish our own residence.
Decades even centuries ago living in the same house with your relatives usually meant other women were breastfeeding their babies and could help out with feeding yours if you were not available.
All ways of living have good sides and not so good sides. My hubby and I still argue about how much my family is involved in our lives and yet he acknowledges when we need help that they are there for us.
I wish on this Christmas Eve 2012 that your baby grows up with the love and care of both families and learns how important a healthy family is.
Good luck with setting those limits. After 18 years its sort of getting easier. It won't take you 18 years, I don't think. Just me.