I applaud this approach, devs trying their utmost to hold everything back(sometimes past it's natural progress) in the hope that things will become more meaningful (or just straight milking) is pissing me off lately.
As you say sex isn't the end.
The sister didn't spend much time in that "we shouldn't cross this line" place, the aunt is still hesitating but also seems very close.
Same thing with the blonde model, seems like she already took a liking to our guy, no family concerns here anyhow and things can happen at any minute...
The mom is the one that will probably take slightly longer, i feel, more mistery and past troubles here.