I kinda don't want to have anything to do with her son b/c she will make my life a living hell.
If your M does get sorted. The absolute very first thing I would do is have LOTS to do with the S. Become the other mother. It is your H child. She would not make your life living hell by doing that, YOU would make hers!!!!!! she would be so jealous that you are able to have a relationship with the child. It would also be easier for your H to manage y0u and the son if you became the boys second home.
Lets look at the relationships
H -- He has you, OW and Son - Juggling 3 and struggling OW - H and Son she is sweet You - H hurting - So add the son to your life
You could become the person responsible for picking the child up , having the fun with him, supporting him, teaching him. The child is innocent in all of this.
