It's A Dilemma

Nearly three weeks ago my step daughter Ally shouted me from her bedroom. When I entered having run upstairs, she was lay on the bed naked, legs spread wide apart and was masturbating using her fingers.
I stood utterly transfixed and couldn't help but get an erection.
Ally made herself orgasm then told me she wished it was my cock that had been sliding in and out of her pussy.

I quickly made my excuses, entered the bedroom I share with her mom and stroked my throbbing cock until I came all over the floor.
Downstairs twenty minutes later after I'd cleaned up, Ally acted like it was perfectly normal for her to be naked and masturbating in front of me. She even kissed my cheek telling me when she gets back from college in a couple of weeks, she expects me to fuck her.

Now here's the head wrecker. Ally has been back home two days. When she was at college, she not only sent me naked pics of herself, but also sent one to her mom by mistake, but with a comment meant for me.
Her mom spoke to me that evening saying she thinks Ally has the hots for me. She then said this "I'm away the weekend after she gets back. Maybe you two should sort out your relationship".

My wife didn't add anything that day, but she did mention to Ally the day she got home, she's visiting her father over the weekend and could she 'look' after her step dad.
The way Ally's mom said it, to me it sounded like she was inviting her daughter to have sex with me.

Obviously I didn't come right out and ask, just in case I've got it all wrong from my wife's perspective, but Ally hasn't stopped flirting with me since.
She even sat opposite me last night playing with her pussy inside her shorts, as her mom was only feet away.

My head is all twisted around. Do I talk to my wife before she travels at the weekend. Or do I just risk fucking my beautiful twenty year old step daughter, as she's already suggested I do.