THIS IS NOT A TROLL: You could always break your dad and his GF up, that would not be creepy at all, you merely feel in love with the daughter of a girl your dad dated, there is nothing wrong with that.
Does she look at you the same way?
You don't want to make it awkward if you make the wrong move after seeing her everyday and your living with her.
You are definitely not a creep. She is not related to you so this is not frowned upon although I would not suggest it.