Max
Well I don't think it was fair of her to expect you to not make plans with Mattie on Christmas. You didn't abandon her, you just...left her to deal with her own family issues this year. I'm sure she won't be upset with you for long.
Maybe you can send her an owl on Christmas to show that you're thinking about her. Or, see if you can make arrangements with Mattie so that you can make a pit stop at Melania's in the morning before going to her house, if you really feel that guilty about it.