I suppose the stupid old man doesn't want to acknowledge her anymore. He only cares about Hawk, the one to raise their family into nobility. Nothing else matters. In fact maybe he's displeased there will be a loose branch in the next generation. After Hawk becomes a nobleman, and the father himself dies of old age, Mary will remain behind as a commoner. It was probably already difficult enough to find a destitute noble family willing to take Hawk in. To find a second family to take Mary in could be beyond the old man's reach. So, he ends up rejecting her.