I tried to calculate how old your oldest was, but I couldn't figure it out. First of all, it's really nice and lucky that your parents helped you when you needed them. I would assume they helped out of love for you and love for their grandchild. Assuming this is true, there is no reason for you to have to do things now in order to "pay them back" for the things they've done in the past. Sure, it's awesome for kids to have a healthy and close relationship with their grandparents, and it seems as though you are trying to help facilitate that relationship. Good. I think you are doing the right thing. You allowed both sons to come visit (I think it's sad that they won't take the daughter, which I am guessing is because she's isn't "their" grandchild-hope I'm wrong on that one). I do not think you should feel under any obligation whatsoever to let him go again. One week long visit anywhere during the summer is enough away from home time for anyone. Aside from that, you mention he's grounded for lying and stealing. Allowing him to go to his grandparents is allowing him to break his grounding to do something fun. Would you allow him to go with a friend on vacation somewhere fun while he was grounded? If you wouldn't, I don't see any reason why you shouldn't be firm and say no on going back to the grandparents' house again.

Also, you don't have to constantly repay past kindnesses or pay for past mistakes. You seem as though you have moved on and are on the path of a good responsible parent. Your parents fostered your son at a time when you needed help, but they don't have any right to claim his time and have no right to harass you into letting him spend time there, especially if they allow him to break the rules you have set for him.

I wish you luck and strength dealing with this!