A few questions (Spoilerish)

I must have missed the condom scene, but (2) Yes, Isabelle's mother was having an affair with the man Isabelle babysat for. Isabelle saw them canoodling during intermission at the theatre after running into her john, Georges.

3. Isabelle is Patrick's stepdaughter, not his daughter. I don't think he had a sexual interest in her. When Isabelle was testing him later, he seemed like he very clearly knew his boundaries and was only letting her go on to kind of let her get it out of her system. I think he wanted to allow her some leeway to "experiment" and realized he could do more damage by being too reprimanding like her mother, thus pushing her toward more bad behavior. When the mother came in, she saw just enough of them flirting to make it look just on the edge of compromising, and I think the filmmaker wanted us to have similar doubts about Patrick's motives with that shower scene. But he's a straight, human male and not her biological father or legal guardian, and Isabelle is, as the title says, young and beautiful, so of course he might take in more of an eyeful than he would of his younger stepson masturbating under the covers. Doesn't mean he would necessarily ever come on to Isabelle. Plus, as an adult, he understands how embarrassing it is to an adolescent getting caught doing what Victor was doing. It's a less traumatizing kind of embarrassment just being seen naked in the shower.

http://www.ScripTipps.com/

She was not her biological daughter. A sexual encounter -- either consented or not --happens (unfortunately) everyday mostly into the country in South American countries. It happens in Manhattan, too... doesn't it, Woody?