How can I keep a young'n from doing something stoopid?

My nephew is moving to be with his twin sister. She was the victim of spousal abuse, and called her brother to ask him to come stay with her for a while. Husband is in jail, and out of the picture for the time being, but I’m afraid that when he gets out, nephew will want to take matters into his own hands. Wife and I raised these two since they were seven, and biomom is out of the picture.

Niece called wife today and told her the situation. Wife made arrangements for nephew to get flight, etc. While wife is at work doing all this, nephew asks if he can borrow Dremel to sharpen knife! Loooooooooong talk. Did not allow nephew to borrow sharpening tool, wife did not allow him to pack knife. My last words at airport were, “Good luck! I love you. Give my love to your sis. And don’t do anything stupid!”