I think she has been playing a role every time. The coquettish woman on the bus for John, the John copy (gun, cane, running around London, eating together) for Sherlock, the therapist for John. She has been playing them like Irene did with the Holmes boys, another Moriarty parallel btw. 
If the last scene reveals the true Eurus, I fear she is a psychopath who is a very good actor at the same time. 

SusiGo wrote:

I think she has been playing a role every time. The coquettish woman on the bus for John, the John copy (gun, cane, running around London, eating together) for Sherlock, the therapist for John. She has been playing them like Irene did with the Holmes boys, another Moriarty parallel btw. 
If the last scene reveals the true Eurus, I fear she is a psychopath who is a very good actor at the same time. 

Wow, very well put. She impersonated exactly the characters both John and Sherlock needed the most at any particular moment... She can read into people.

I think she tells John about the night together, because that's what John has heard Sherlock say about Culverton's daughter ("we had chips").  She's revealing her other personas/disguises.   And I agree Susi, that she was creating characters that would appeal to them.  Apart from the cane, her version of Faith is nothing like the real one (not even in dress), but seems more vulnerable, more genuine, oddly enough, and a damsel in distress that Sherlock would want to protect (after failing to protect).   

What's really clever is that I, as a viewer, found fake Faith and E on the bus appealing as a viewer - they were both so likeable!  I was reluctant to believe E was a baddie, even though I knew she had to be in disguise.   So very well played.

with what she implied about the fate of the therapist, I fear Eurus is more than just a bit unbalanced. But that does not in itself exclude that she does in fact like Sherlock. As Irene Adler told us, a disguise is always a self-portrait. So not everything about it needs to be fake. Might very well be that she was in fact surprised by Sherlock's warm and caring side. Not surprisingly, if we assume that the only brother she had any contact with in the last years appears to have been Mycroft, who's caring can be quite unsettling for the affected parties. 

They were preparing her arrival from the beginning, for years. I hope, really hope , that there is much in the story, not just a killing psychopat, nothing is just black and white. Sherlock finally after years starts to show and understand positive emotions. Where they working to this point just to have better means  to ruin him? 

I hope not, hope that something positive is going to arise from this family history both for Sherlock and Mycroft

I'll second what Susi said. 

A 'good' psychopath or sociopath are good at playing the 'likable' card... and especially the pity card too. It takes experience to see through their 'acting' and usually they have a lot of people around their finger... 

And well, Sherlock was high... so he was easier to manipulate... 
 

Good gravy, Sunday has to come quickly. This whole sister thing has distracted me all week!

In the Sussex Vampire the boy tests poison first on the dog and then tries to kill his sibling out of jealousy. they might use something from it. 

Jealousy of children can have any reason but the most often  older kid is jealous of  newborn or some coming newly into the life. Sherlock was old enough to remember something so was it long term or is he adopted? 
Or probably something totally different 

I've been thinking a lot about The Sussex Vampire and I wonder if Sherlock is more likely to the poisoner, as presumably he's older.  It's a while since I read the story, but I seem to remember the mother has remarried, so the younger sibling has a different father.  That couldn't possibly be the case with a younger sibling of Sherlock's as his parents are so clearly his parents, due to the casting!  But also, Sherlock is a chemist (poison). 

I know Eurus putting an act doesn't preclude her genuinely liking Sherlock (and I suspect she might even turn out to be not so evil as it appears), but I'm still bearing in mind that she seems completely genuine as E too, and appears to genuinely like John.  She's just a brilliant actress, so we can't be sure of anything yet.

Maybe it won't.  Maybe, come 2020, there'll be a 5th season!