The housekeeper has TB (hastily written into the storyline because actress Jean Marsh suffered a stroke in real life).
Eileen Atkins, who played the mother, didn't want to appear in series 2, (apparently had issues with the storylines) so Blanche's character was introduced instead and the mother was killled off.
I have to say, I thought this week's episode a vast improvement on the previous two. Much less of a history lesson and more character based. I wish they'd get rid of the Duke of Kent, though. He seems to be shoe horned into every episode and adds nothing to the series.