Sigh, I really dislike it when a poor animal suffers due to situations hoomans create for themselves...

It sounds as if Mavrick needs to be put in a home that will spend time with him. You'd think with the husband being home so much he could take the time to train him or at least provide for his basic needs. No offense to your sister or anything but I think that they need to solve some personal problems before taking on the responsiblity of caring for an animal. Maybe you can convince Brandon the breakup will be so much of an issue it will force him to re-home Mavrick, then you could swoop in and "save the day" and help out, and no one in that situation could be realisitcally mad at you.