I would suggest you get legal advice on this. I bought my house with my mum many years ago. Like you, I had no chance, and as it was a council house (in her name) we bought it. Anyway, I have a sister too, so I was a bit worried (especially as I was paying the mortgage!) that when my mum died, my sister would by default get a share of the house I was paying for and living in. I actually got my mum to specify it in a will that her share of the house would pass to me, but I was told after - don't know how true - that it would have come to me anyway. As I said, I don't know if that was true or why if it was but for safety's sake, get proper advice now to ensure it goes how you want it to.