Sorry about the gap, something dropped out and I hit the 'post comment' button accidentally.
Thing is, a grandma can have good influence on her grandchild's life by doing so directly, not by telling her grown child to do this or that. You can spend time with your grandson, doing something fun together without anyone else. Go get an ice-cream cone or go to the park. If he roars or pops up with some other behavior that makes you uncomfortable about his manners, gently correct him. He will behave when he is with you if he wants your approval. Make yourself so popular with him that your approval matters to him. That's not hard, with a toddler, all you have to do is love him and listen to him and play with him.
If you are truly bothered about him sleeping in the bed with his parents at your house, you could try offering them a blow-up mattress for him to sleep on. However, now that it's a loaded issue, your daughter might not appreciate it. If you think there is something sexual going on at the other house, tell your daughter this, and leave it to her to assess whether he should be spending the night. She is the mother, not you. Even if she is staying in your house.
It didn't really sound like you thought he was being molested or witnessing sexual behavior, though. I'd leave it alone, he's only 3 1/2, and probably simply loves to cuddle up and feels more secure that way.
Regarding the other family roaring all the time and him acting that way, he's going to pick up undesirable behaviors all his life, from friends, colleagues and family, and you really can't control whether he does. My son is 4, and his present favorite insult word is "poop," and he got that at preschool. Do I keep him out of preschool because I don't like the language he hears, or do I accept that as the price of him having the experience of being around other kids? He's getting a lot out of preschool, and though I might wish it doesn't include learning the words "damn" and "poop," this is not the last time school is going to have unintended learnings. :)
As for the changes in behavior you impute to his father's side of the family being bad role models, all you can do is tell him that you don't want him to behave that way around you.
I understand what you mean, however I think at 3 year old its perfectly okay for a family to have him sleep with them, many families do.As he gets older in my opinion his own bed is preferable , for a quality nights sleep. As his parents do not mind and seem comfortable with the arrangements it may be aswell for you to let go and not be so concerned , as this could be a temp arrangement it will be solved for you .You cannot control what is done in the other house he goes to.,all you may do is cause anxiety amongst family.I realise that you have your grandson's interest at heart.