Watchin' the detectives...
Hm. I just don't know.
I have quite simply never bought that Joan is/would be attracted to Mycroft. Certainly not in any relationshippy way.
Not only does he ooze deviousness (which, honestly, ought to put her detective hackles up), but I've just never bought that she wouldn't know or care how much it would bother Sherlock, or worse, would know, and would just steamroller ahead anyway. (You may say that she wasn't steamrollering, that she basically told Mycroft 'wait' - except she's clearly slept with him on multiple occasions - I don't care if there's no formal arrangement, that's steamrollering right along.)
I just...after the first season, and naming a bee after her and all - I felt that she had a pretty good grasp of how important she was to Sherlock. Whether she should be or not - perhaps he's developed an unhealthy attachment to her, using her as a crutch, because she *was* there as his sober companion and helped him very much. She would still know/see that, or should, but I can't think she'd be so cruel as to ignore that, and hurt him so directly by taking up with his brother - who she *knows* he has issues with.
Sherlock even mentioned the 'certain intimacy' that they had - apparently Sherlock's the only one who feels it/is trying to honor it. (Although she's not wrong that he has invaded her privacy....at the same time, she hasn't been forthcoming to him, in respect of that supposed intimacy.)
I also thought the heroin stuff was a little too 'easy' this ep, too. He's super-duper aware of his sobriety and the threats to it at the moment, with Alistair and all...he's making conscious *decisions* to move toward the drugs, and I am just having a hard time with that part of it. Unconscious, maybe. Because he's emotionally raw. But he *knows* he's raw, and in danger, and I just don't see him doing that *consciously*, the way he did.
What totally blows, though, is - Joan never seems to realize the effect that her pulling away from him has on him. This time, with the hiding drugs, previously with the secret letters to Jamie. Every time, Joan's separating from Sherlock does a number on him. Now, I'm not saying that she should be *responsible* for him, or curtail herself simply because Sherlock is weak - except...it's like she doesn't even notice, or care. And I feel that she ought to notice, at the very least, and they should have some serious conversations. But he avoids talking, and she avoids depth. She pesters him, but it's all at the surface level. She's not nearly as in-tune with him as she claims to be. I feel like she doesn't *really* listen to him, until he unloads something serious in an attempt to keep her wandering attention. To keep her close to him, because she's drifting away. :-(
Last season's Joan didn't seem like that kind of drifter - she seemed to be paying more attention. She seemed to care more. I miss that.