I also believe that something happened during their childhood, and that it is linked to Redbeard.
I like the idea that Sherlock believed her to be dead (maybe she and Sherrinford are two different persons and that Mycroft believes her dead too, but I really doubt that. I mean, he's Mycroft, he's omniscient! lol). Which would explain why he doesn't recognize her. (he's Sherlock Holmes, if he still was in touch with her sister, even high and even if she was in disguise, he would have recognized her!)
And I think Sherlock might have unconsciously recognized her during their stroll together. He has a childhood flashback just before she "disappears" (how does she do that, by the way?? She's nowhere to be seen when Sherlock turns around. Surely, she just can't evaporate like that!)
For some reason, it made me think of a book I read a couple years ago: "Mycroft's testimony", I just found the post I'd posted here about it, here's a quote:
"Mycroft explains that Sherlock had a twin called Arthur and who the Holmes family called Arty. Arty was evil, and he was really bad to Sherlock. Sherlock was literally his punchbag.
When the twins were about 7 year old, Arty was found dead, floating in the house’s pond. Mycroft found Sherlock in tears, a few hours later, huddled up in the outhouse, terrified, swinging back and forth, repeating « Mori Arty » (mori, -ior, -eris, moriuus sum… here it could simply mean « Arty is dead »)
Although Sherlock claimed it was an accident, his mother was convinced he had killed Arty and from that moment, despised him. "
Well, ok, it sounded like bad fanfiction but the book was actually good.
But it could be something like that. Either she did Something bad to Redbeard. Or there was some accident (which happened in the memory that's been haunting Sherlock and that we have been shown several times now) and Sherlock thought himself responsible, which would explain a trauma and all that. And it may also explain why Mycroft became so overprotective.
The "Miss Me?" is very intriguing. I wouldn't be surprised she is linked to Moriarty indeed.
And if Sherlock thought her dead, or at least, even if he didn't think her dead but hadn't seen her for years, the "Miss me?" would really make sense! Maybe it's something Eurus had told him when they were children.
And maybe Moriarty used that as a hint to Eurus...
Awwwww... I can't wait!!!!!!