I'm thinking Dad's got to have a wet dream any night now. And/or, he's going to have so much pent-up spunk that he's going to have to jack-off one night, and since the bathroom doesn't have a door and they feel pretty comfortable with nudity, she could come hurrying into the bathroom, only to catch Dad shooting his wad in the shower or on the toilet.
Once either of those happen, I figure Dad is going to have to come at least partially clean about him being a normal, heterosexual man and her arousing him, and that'll give her a lot to think about (she's already expressed a sexual interest briefly to herself), and then all she has to do is say, "Dad, since you are teaching me how to be a good girlfriend and you've admitted to being aroused by me, do you think you could teach me a few things about petting/sex?"
And then it's off to the races.