I'm not aware, but technically speaking of course brothers should get on alright, especially since their mother AND sister died not so long ago. It's a ridiculously stupid idea to cut ties with your brother and the thing only young and stupid and arrogant can afford.
Music-wise I don't think there's a chance of them working together, they're just too different and have a too similar voice anyway. Mark still is not a kind of guy who let anyone be his co-writer, so what he can do with David? If not singing, if not writing, then it's basically nothing.