I can see why you are concerned.

If your mum knows of the issues but hasn't done anything about it, then the best you can do is attempt to instill change when they are with you.

This means taking the tough stance:

1) Tell them TV is on until 10am and then they are to go outside to find things to do. If it's raining or too hot etc, find age appropriate activities indoors and explain that the baby's toys are for the baby.

2) When you discover urine on and around the toilet, call both boys in and calmly ask them who did it. If one confesses, then they MUST clean it, if neither confess, then they BOTH clean it. Tell them it isn't fair that you should be the one to clean after their mess or find the bathroom in that sort of condition.

If they do it again, tell them they must now urinate with the door open so you can make sure they are peeing properly - emphasis being on the fact that if they either cleaned up their accidents OR took more care, they would earn the privilege of privacy.

3) Try to have discussions with them one on one to see if there's a notable difference between when they are apart and when they are together. Try to talk about things of interest to them (even if it seems infantile) and explain to them that baby talk at their age is just going to cause them grief amongst peers when they move (if they are using this deliberately)

It seems like your mum has babied them beyond belief and because of that they act the way they're treated. When you have them, I'd take it upon myself to show them what they can do at their age and reward their good behaviours.