This type of behaviors is not good for the children......it teaches them to be suspicious of everyone and to run back and fuel gossip and trouble. Sounds like mom is having trouble understanding that people do talk, that is a simple part of life and while sometimes the things said are not always nice it isn't always meant to hurt anyone.
This is very unhealthy for the boys to say the least. You may want to sit down and talk with the mother and let her know that the boys should not be used in such a manner as it is not healthy for them to consistently be used in such a way. Eventually these boys will come to resent her for this as they will begin to think that they are the cause of the problems in the family and may begin to pull back from the whole family in a attempt to stop the messaging.
If you do not feel comfortable talking to mom, consider talking to your son. Let him know this is not good for the boys and makes you uncomfortable to be around them. Explain to him that the kids should not be used as pawns in a game such as this and you do not wish to be a part of it.
You may have to sit back and only be with the kids when mom or dad is there to get your point across and show mom and dad that this is not acceptable and you will not continue with it.
I know this is hard, but the mother really needs to open her eyes and see that what she is doing is only harming her own children.