Sounds like the house you bought with your sister is an investment property for you and not your PPR. If this is the case then tThis would have several implcations including (a) the joint purchase would not have been considered a first time buyer purchase (which would have stamp duty implications) (b) you would not be entitled to owner occupier [broken link removed] (your sister would though) (c) CGT on any gain arising on your share of any resale gain in the future etc. You can claim [broken link removed] for the rented accommodation but you can't claim rent relief and owner occupier mortgage interest relief at the same time or if the property is an investment property. You might need to get independent, professional advice on the implications of your situation.