Margaret Weylin is similar to her husband Tom insofar as she has crazy mood swings. Her approach to their son Rufus, on the other hand, is just the opposite. While Tom is a hard taskmaster, Margaret uses every opportunity she can to spoil Rufus rotten. She gives him whatever he wants and it's little wonder that Rufus never really grows out of his childish need to always have things his own way. The one thing that sets Margaret off with Rufus is when he asks her about Dana disappearing into thin air. As Rufus says, "I asked her where you went […] and she got mad and said she didn't know. I asked her again later, and she hit me. And she never hits me" (2.2.51). It looks like Margaret Weylin is really, really uncomfortable with things she doesn't understand.
It's not surprising that Margaret's treatment of Rufus contributes to the horrible way he treats his slaves (and especially women) when he gets older. At one point, Dana thinks, "I remembered suddenly the way he used to talk to his mother. If he couldn't get what he wanted from her gently, he stopped being gentle. Why not? She always forgave him" (5.6.85). This kind of treatment leads Rufus to believe that women should always give him what he wants and should immediately forgive him whenever he does something terrible to them. So yeah, Margaret Weylin hasn't been the best mother in the world, even though she loves her son so badly.