Mary only appears in one scene in this movie, and it's one of the saddest scenes we get. After falling into a huge fortune, Mary has decided that her son Charles should be raised away from her and her husband so he can have a better chance at being cultured and well educated.
When Mr. Thatcher asks if her son's ready to leave, she quietly answers,
MRS. KANE: I've got his trunk all packed. I've had it packed for a week now.
She's obviously very sad about what's happening, but she can't bear to have Charles grow up in a house with her abusive husband. As she says:
MRS. KANE: He [Charles] is going to be brought up where [his father] can't get at him.
What she doesn't realize is that taking a boy from his mother is also its own kind of violence. If she'd known how much this decision was going to mess up her son, she might have reconsidered.