Is it normal for grandparents to be obsessed with grandchild?

I am a mother to three kids, 9yrs, 6yrs, and 3yrs and my parents will not stay out of our lives. They are constantly invading my privacy and trying to tell me how to raise my children. My oldest Mikey is absolutely spoiled and gets whatever he wants. The two youngest don't barely get ANYTHING from them, and they don't care if the other two know what they've bought mikey.. They let mikey stay the night at their house every time he asks (If they had it their way, he would move in with them.... ) He stated that he was going to use their living room as his bedroom and sure enough IT HAPPENED! The other two can't even bring a toy in the living room, they are forced to a confined area for play time. Mikey gets to eat dinner in my parents bed, or in the living room but the other two can't. if they don't finish dinner, they get no treats, and majority of the time Mikey doesn't even touch his food and gets Candy, Sweets, Ice Cream,etc. They won't ever keep the two youngest ones for more than a few hours. If they all three stay the night, my mom will return the two youngest to the house at 6am but make an excuse as to why Mikey isn't here with them. Christmas came, and with no shame they gave him three times as many. I went to pick Mikey up from their house on Fathers day morning and they freaked out on me. Screaming, yelling, degrading and belittling me -- Even to the point where she sstated she was no longer a mother to me, and it is Janet not mom to me. I've done everything in my power to get this to stop, but it is beyond my control at this point. My husband and I work out butt's off to get the two little ones anything they want because every time Mikey comes home he only has ONE instead of THREE of whatever they bought him like their should be.I've repeatedly asked them to stop buying him stuff, as he doesn't appreciate ANYTHING .. a corvette would probably only make him grin a little bit.. That's how bad it is. Everything in their house Mikey calls his own and they agree and tell him that its true.. It's one step forward and two steps back with them, any and every bit of discipline I excercise on the children is completely ruined when one day is spent with them. One last thing, Mikey is ADHD and was on medicing for about a year before I took him off of it and he lost ALOT of weight... Two to three months after being off the medicine he had already gained 25 pounds. I ask them to stop buying fast food for him and i woild either cook and freeze his meals or bring the food uncooked for my mom to do it.. but if he doesn't want what is cooked they will go to no end to get him what he wants. They live 30 miles out of town and they will come into town at night/morning/ any time of day if Mikey wants to go out there or even if he just wants to go to the store. The worst thing was when he left his psp charger at their house and they came all the way in to get it for him. I just don't know what to do and I need some serious advice. Is this behavior normal?