I have a similar sort of problem. Although, I've had a huge falling out with mine so my children and I don't see them - but my husband still does. When my dd (youngest) was born my MIL carried on about my "poor" ds and how he was going to be so left out etc etc. I was really offended - I love my son and would never favour one of my children over the other - my MIL blatently shows favourtism with her own kids so obviously she thinks everyone is as shallow as her. Anyway - enough of my griping and back to the point. On many occasions my MIL has sent quite expensive toys home with DH for our son and nothing for our daughter, I know it's because he's old enough to be "bought" and also because she assumes I'm such a horrible mother. Today is my DD's first birthday and she sent home a small present for her and a heap of presents for my DS - and it's not his birthday!!! I don't talk to her so I'm not going to say anything yet and my 2 are too young to realise what's going on, but if she keeps it up I will be saying something.
I don't know what your relationship with your inlaws is like, but if you get along with them maybe try and be diplomatic and just explain how you feel, they may not even realise what they're doing.