She's not my kid...but yeah, I still try to be a good role model, and I'd
actually preferred to just be honest with her about it when she first seemed to suspect, which was at least a year ago. We talked about it back then, at which point there was something going on, and we were worried she'd see something and think her stepdad was cheating on her mom. But her mom seemed uncomfortable with talking to her about it, and as the parent, it's her decision.
By now, there's nothing going on, and the text she saw was about avoiding situations that could turn sexual, and relying on willpower, so I think that was pretty clear. I talked to her a little today, and she's suspected for awhile, but thought we'd lie to her if she asked, and when I asked if she'd talked to her mom, her reply was "Sure, I can see that 'Mom, is your husband having an affair with our housemate?", to which I pointed out that it's hard to get anything past her mom, and her stepdad and I really suck at being sneaky or lying. Then later on when her mom was home, we both talked to her about snooping, and that while she's a teenager and her parents pay her phone bill and her mother has a right to know how she's using her phone, I'm an adult and she has no right to invade my privacy.
I'm not sure if it's all perfectly clear now. I really didn't feel comfortable deciding how much to tell her without talking to her mom, but I didn't want to lie either, so I pretty much asked questions and affirmed what information she'd already snooped up: Nothing is going on, but there is mutual attraction, and that's sometimes hard to deal with. And there's no way we'd even be capable of sneaking around behind her mom's back, so if she wants to know, she should ask her mom.