What state do you live in?It differs from state to state, but I know for a fact in my state this would be deemed coercive (Just due to the fact that he is a landlord and it is illegal for him to make, let alone carry out an such an offer) If the offer were turned down this could be deemed sexual harassment, but since the arrangement has been carried out it is most definitely rape. Since the victim in this case, may potentially feel pressured to take the offer because she may feel she has no choice in the matter, i.e. if she doesn't the land lord may evict/kick her and her brother out. In the eyes of the law this is illegal and any coercion at all in regards to sex falls under the umbrella of rape.
Even if she is not physically forced to have sex with him, she is still be victimized by a sexual predator.
I just took a course on sexual violence and we covered scenarios just like this. If she wanted to she could easily press charges.