Hi Nams- I think that you deserve to say "no," and that you don't have to have a reason for it. It sounds like you've tried to talk about it logically with this child's mother, and she can't or won't hear it. If I were in that situation, I would probably phrase it as something like this "I know that your son has problems from his injury, and I wish that I could be more supportive to you and to him. However, he has threatened my 4-year-old, and I need to be protective of her first. Maybe we can pick a time for him to come over for a hour or so when she will not be home?"

You don't have to say no "forever" but set boundaries..whatever feels okay to you. And if that means he never comes over, or he comes once a week for a hour, whatever--it is okay.

I know setting boundaries is SO hard..but this sounds like a great place to practice them :-)