B.C.
Siblings don't have to like each other, but they do have to be at least as civil as they would be to a stranger.
No name calling.
Divide and conquer - send them to separate rooms - if they can't play nice together then they don't get to play together at all.
Also - privileges (phone, tv, computer, video games, etc) are all constantly earned by good behavior (getting home work done, getting chores done, being respectful to parents and each other).
Bad behavior means a privilege was not earned and if they lose it every time, they'll learn to get their act together sooner than later.
You and husband (and hopefully his ex) should be on the same page discipline wise.