I really think that you should see a social worker that specializes in family counseling... Even if you are the only one that goes. I think they will have good advise on appropriate ways to discuss this whole situation with the children.
Sounds like those boys have been through a lot. I know you want to do the right thing by them, but I believe it is usually recommended that parents discuss what is going to happen together or with professionals around if the situation is volatile.
It will not be in The children's best interest for them to feel they are choosing or confiding in one parent over the other. I know you want to be honest, but there must be a different way to approach it. Professionals will be able to help.
I think this is the most important thing to figure out right now.
