I would have said that she looks too young, but actually there's only a few years between the actors, so it's possible.  Yes, Mycroft did say something like that.  Which could have various meanings - that she influenced him in a major way, that he was affected by her loss/"death", that Sherlock was treated differently because of her, etc.

Pops wrote:

He said exactly : "every choice you've ever made, every path you've ever taken, the man you are today is you memory of Eurus".

Where is this sentence from?

I've been waiting for twins ever since TAB.  Just wasn't sure who would be a twin of whom!