My mother-in-law favors one grandchild over all her others, how do you handle this?

Before my husband and I married, he and his son lived with his parents. (He did this because he worked out of town frequently and also wanted to keep his son in private school.) My mother-in-law frequently brings up the fact that she practially raised the now 13 year old. The problem arises that she treats the one she helped raise differently than the rest of her grandkids....which is understandable....but still difficult to address. She has two other biological grandchildren through her daughter that she treats differently and then there are my two that she actually treats better than the other two biological (which can make things a little awkward). However, the 13 year old that she helped raise...She constantly buys extravagant gifts for him...for no reason...not birthday, etc. An mp3 player here, oh did you lose it? let me buy you another one. How 'bout an $180 video game (RockBand) and then another $50 video game in the same month? Then, she buys him clothes. In all fairness, she does buy these things for him when he stays over there...he likes to go over there on the same weekends my kids visit their dad (my kids' dad doesn't but them squat by the way and my parents are not the "spoiling' type). However, the 13 yr. old brings these things home...or my go over there and see them...and then they want to know why they didn't get anything. My mother-in-law is a sweet person...she's not being spiteful...she does buy the other kids birthday presents or little things here and there but they in no way compare to what she buys the 13 year old. I just don't think she gets what kind of position this puts us in (or me in, depending how you look at it). My husband has talked to her about it a few times before...she backs off...and then it starts again. If I try to make this up to our kids...then the 13 yr. old feels it isn't fair...(which we could never spend that much anyway)...or, we can not try to even it up and then the other kids feel slighted....I haven't even gone into how this makes my sister-in-law feel. My mother-in-law can barely remember when their birthdays are (the other two grandchildren). I might add that any time I have to bring this up to my husband, I feel like he thinks I'm being a *****...I tried to say this to him in the most pleasant voice possible and he still says that I come across bitchy. I can't win any way I turn here. Please help. ....I might also add that the 13 year old has no idea what it's like to earn anything...he is taking care of things a little better since he's lived here...but, he knows if he breaks something...she'll buy him another one...and if we won't buy it...next thing we know...she's done it. Or if we don't buy him exactly what he wants for his birthday...she makes it up to him the following month. Thus, the Rockband video game....we bought just the drum set which was $90....he wanted the whole thing. He got it the next month when he was at his grandparents' house. ?