I think they’ll explore that more in the upcoming episodes, we know Margot sticks around for at least 3 or 4 more episodes, Mason’s first episode is 2x10.
Mason is abusive, he broke her arm and he literally drank her tears, and Margot doesn’t believe he’s changed/going to change, that’s why a not very slick Hannibal strongly suggested that she should kill him or get help to kill him, by the way Martha De Laurentiis teased on Twitter “If you really want to kill your brother Margot, wait until you can get away with it. Or find someone to do it for you. Enter Will” ;)
In the book Margot was raped repeteadly and abused by Mason when she was a child, but Bryan said women won’t be raped in his show so I wonder how he is going to handle this story..