I agree with what Live said earlier (have a chat with sister) or just send her a text if u would be more comfortable with that,keep it short and to the point tell her what he said and how you felt when reading his text anxiety due to your history with him (just wanting to make sure someone is aware of what according to him he is doing)and thats it leave it at that.Dont contact him. All you are wanting to do is know for your own peace of mind, that you have made an effort to keep anyone including him from being harmed.Thats all you can do he is a grown man,you cant control his choices and you cant pay for his mistakes,they belong to him.Im so sorry you've been put in such a horrible place having to make these decisions its so un fair.
Dont keep a secret for him