The girl who is afraid of being touched is a nice touch (yes, yes, groan all you want, I couldn't resist the pun, sue me).
I do agree that the writing seems a bit childish, like the renders got all the care and the story was slapped on at the end. The older sister's personality is weird ("I have a boyfriend! Feel my ass!") and she looks old enough to be the mother's sister. The MC's amnesia fluctuates wildly - he remembers people by name one minute, then can't remember how the younger sister liked him for years in the next. Still, plenty of time to work on all this, it's just the demo after all.
I think it would be really cool if our MC was actually faking his amnesia to create opportunity for himself getting into the family, like a "Oh, I don't remember any of our baggage or our relationship before, let's start fresh. Wanna make out?" type of deal. The player would have to balance the other people's suspicions against using what the MC knows about them to seduce them. A twist like that would really make this game, dare I say, Something Special (alright, enough of the groaning! It's just a fucking pun!).