I don't think that you should have to admit to every bout of flirtation that you get into. But the following details make me think that, in this case, it might be better to at least mention it to Davis... and even to Jay, so that he knows that your boundaries are not
going to be crossed.
I haven't felt crushed out quite like this in years, maybe not since high school, in this unacknowledged and unacknowledgeable way. [...] I'm positive, as of tonight at least, that it can't just be me, Jay has to have noticed it too, this dynamic we've slipped into. Hell, Davis has probably picked up on it too, he's an empathic sort of guy. [...] And in the meantime... masturbate furiously... er, I mean, cold shower...
I wish this blog weren't a secret and this crush weren't a secret so I could post pictures of Jay. You know you're into someone when you want to show them to everyone you know.
Seems to me that - this time - your feelings for another are stronger than usual. Flirting can be even more fun / exciting when you DON'T know whether it's going to lead anywhere further. But it can also be fun (and much less dangerous) if everybody concerned (You, Davis, his sister, Jay, and Jay's GF) KNOWS that it's only flirting and no more. Given your own polyamory, Davis' having to deal with his discomfort at that, his depressions, and his sister's mistrust of you, you'd be playing it safer if everything were clear and above board. Things like this may seem unimportant to one of the parties (so they're not talked about) until one of the other parties springs the surprise on you that for the last 6 weeks jealousy has been gnawing at their entrails...
Especially given that Davis might have picked up on it by himself, I think that you should talk about it.
But, as opalescent
points out, don't make a heavy deal out of it.
And keep taking the cold showers... or whatever.