My first thought was, his poor little sister! Good for you for taking a second to just walk away and breath...wow! Such a hard decision when you have siblings....in all likelihood if you term one both with go. Big loss of income. Bit what a relief it would be. I can't imagine always being on edge wondering who and what he may harm next. I would definitely give a warning. This just cannot go on happening, for your sanity and for the other children (who just may pick up this behavior)...for safety of all! Hopefully with a warning Mom will realize the severity and come on board with helping this little guy overcome these behaviors.
My strategy when something major occurs (hitting etc) is immediately they go in a booster chair for time out (tray attached) and the timer goes on. It is very rare that I have to use this, but it works. Sounds like you've tried similar already, though. Good luck to you!