I think you should say something. I wonder how much your sisters partner is actually paying in mortgage. It certainly isn't going to be �700 a month- that is excessive, so either way, they are making a profit from you. I would tell her what you have told us. Just drop it into conversation, saying "Interestingly, three houses down our road have come up for rent with new kitchens and bathrooms etc, - guess how much they are asking for them?". Hopefully she will guess, and guess �700. Then you can say "Actually no, they're only �600!". You don't need to say anything after this- I would hope her conscience would sort it out with her partner.