Meghan must have known when she and Harry got engaged there was a strong likelihood that her father, who sounds like he has mental health issues, and her loose cannon of a half sister would be problems that couldn't be swept under the carpet. She and Harry could have taken steps to avoid public embarrassment but they pushed him away instead.
I thought he was an irritating attention seeker during the run up to the wedding but surely Meghan can see that a gesture of reconciliation, especially at Christmas, would go a long way to healing the rift.
She will only have herself to blame if the situation gets worse. I wouldn't mind betting the Queen will urge Harry to do the right thing by his FiL. Other in-laws (before the Middletons) have faded quietly into the background but there's no way Thomas Markle is going to do that.