He is an adult, he's 30 now. On one hand, it's terrifying for me. I didn't grow up with an autistic family member like my wife, and although he's high-functioning, he's still very much like a little kid. He gets overwhelmed with his emotions. But I've seen him get violent, and the general consensus is that when he makes a promise he will keep it, even if it takes him years to act on it. But no one in his family is going to contact the authorities about it. If I did that, I think it would destroy my marriage. My wife adores her brother, even with the threat. She grew up with him like this her entire life, and she's seen how unfair life has been to him. She's got a huge capacity to love and forgive, and to her, he will always be her little brother that just wants to have a normal life.
Best I can do is put my foot down and say that once she's pregnant, he can't be a part of our life anymore - until he decides to rescind his threat of violence. But since he's freezing us out already, there's not really much else for us to do except be heartbroken that this is what his situation is like.
No worries! It is really heartbreaking. He does see a therapist, and he has for awhile now, but I'm not privy to how that's working for him.