ALJ do you have any formal/legal arrnagement for his visits? For child support? et?
I am thinking HIS MOTHER is not the best person to supervise the visit. I was going to suggest you have one of YOUR family members supervise his visit. THAT may test his resolve to BE a parent in teh first place ... since you said he aviods your family.
Tell him "the kids are with my sister... feel free to call her to arrange a time to see them" or something like that.
It will be interesting to see how he handles that... Right now he visits with his family and they just coddle him and enable him... that is NOT a HEALHTY ENVIRONMENT for YOU to have to expose yourself to.
If his family wants to do that, then don't be around it. Visit them wehen he's not there... and have the visits with his kids someplace that's more supportive of YOU... right NOW its all arrnaged in HIS FAVOUR and its going to make YOU ILL