All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
In reality, Riba (interest and usury) is one of the major sins and the worst crimes in the sight of Allah. So, it is strictly forbidden to deal in Riba except in a dire need; a need that cannot be satisfied through other ways.
The transaction you mentioned is, without a doubt, a form of Riba. Even if the company pays the interest, this does not make that permissible. Because the prohibition of Riba covers each and every party and transaction related to such an activity.
In this regard, Jaabir, may Allah be pleased with him, narrated that the Prophet said “Allah has cursed the one who consumes Riba (i.e. usury or interest), the one who gives it to others, the one who records it and the one who witnesses it (its transaction).” He added: “All of them are equal in sin.” [Muslim]
No doubt that the relation of the witness on Riba transaction and its writer is weaker than that of the one who concludes the contract itself. So, the latter is more deserving the curse since he committed the sin directly.
Thus, we state that you have to avoid such a transaction. In addition, your sister's state can be solved through renting another house and avoiding mixing with the present owner as well as preventing him from entering the house if any forbidden act is possible.
Anyway, since you can rent a house by avoiding any forbidden act, then you are not allowed to deal with such a transaction as you described.
Allah knows best.