is it possible to have sex with twig
well ... spoiler:
if you are friendly you can't .....BUT........
if you are going for the sex route then its possible with and without thea in the house. If you dont say you want to "free her" and say you want her to be "your slave" and buy her for sex then you imprison her in the basement and train her there as your sex slave... with Thea in the house you tell her a lie that there is a monster in the basement so she doesnt find out...