I've seen it loads on threads recently where people are asking why one partner is buying for the other partners family because it's 'their family, their responsibility' (I'm talking mainly about kids here).
I buy gifts for my nieces and nephews on both mine and DH side of the family.
As far as I'm concerned, all my nieces and nephews are just as much my family on both sides, I don't see why I wouldn't buy for them.