You make her stop...

"Riley... no." You take her by the shoulders and stand her up, pulling free of her mouth with a pop. Her face is painted with upset.
"....but, daddy..." she sounds like she's on the verge of tears.
"Riley... you can't...we can't do that. You are my baby girl, I gave you life, I raised you, I taught you right from wrong, though I clearly failed at that, I can't let you do this."
Riley's eyes well up. "I... I love you, daddy... When I think about you I can't help it! I didn't ask to feel this way..." she starts crying. "The last time Matt visited, I called out your name during sex... that's why he stopped visiting... that's why he was making out with someone else at spring break. I'm never going to have a normal relationship...." She sobs against your embrace.
"Angel.... sweet baby girl... you're just confused. We can talk to someone. Help you out of this phase. It's ok." You stroke her hair."But we can't indulge this, even if I wanted to."
She shudders with gasping breaths. "I guess there's no way I can compare to Momma..."
This probably isn't the best thing to say, but you do anyway, "I shouldn't say this honey, but you are every bit as beautiful...and... sexy... as your mother. A lesser man might have just... let that continue..." you now realize how easy it would have been. She looks into your eyes.
"..y....you're just..s-s-saying that..."
"Riley, I _shouldn't _say that, because I'm your father. That's how you know I'm not lying."
"Thanks daddy.... Maybe.." she sniffles, and you offer your shirtsleeve for her to wipe her nose and eyes. You've been doing that since she was a baby. "Maybe I should talk to someone."
"I can talk to Tyler, discreetly." Tyler is your therapist, and has done family sessions before, so he knows Riley. "I'm sure he knows someone who specializes in sex stuff."
"I... ok...." she hugs you. "I'm sorry daddy. I just love you. And in my heart, I really believe no guy out there will love me as well as you do..."
"I know honey. I know it feels that way. It's going to be ok."
After a few minutes of quiet togetherness, you escort her back up to the house. How very close you had been to disaster. She had felt amazing, and you're sure that somewhere in your fucked up brain you could have justified taking the next step. It probably would have fucked up Riley for life.
You and the girls play card games, board games and talk about school and other things for a while. It's good to have this natural fun with Riley: she seems changed now, calmer. Eventually, the girls start to doze and you bundle them off to Riley's room.
What an.... unusual encounter... never did you think something like that would happen. You feel a little pride at how you handled it... though part of you can't get the image of Riley, locked around your dick, and how it felt, out of your head.
As you shake it off, the headlights of an Uber swing into the driveway.