"It’s a disgrace, sending your little brother into danger like that. Family is all we have in the end, Mycroft Holmes."
I feel like in the context of His Last Vow, Mycroft’s reaction in this scene makes a lot more sense.
I’ve always thought Mycroft’s “Oh, shut up, Mrs. Hudson!” was a strange, and bizarrely strong, reaction on his part. It’s true that he’s capable of being just as callouss and socially inept as Sherlock, but we also know that his position has caused him to develop a certain level of manners and diplomacy. It seems out of character for him to lose his composure just because a sweet old lady chastises him.
Unless she was touching on a sensitive subject.
Imagine that this line in His Last Vow implies what we think it implies: that there was a third Holmes brother, and that Mycroft was somehow involved in his demise.
If he’s harboring guilt about this it’s no wonder he prickles at Mrs. Hudson’s seemingly innocent comment.