Someone talking about doing this on Jeremy Vine this lunchtime.
I may be really dim, but if you do this, with your surviving parent selling their house and joining with you to buy a massive house - what happens, in the future, when this parent dies? Surely their equity is then tied up with your property, so if you have siblings they won't be very happy about the fact that a chunk of their inheritance is basically paying your mortgage now.
But as the person who has taken the parent under your roof, you may feel that you are entitled to this financial perk... and why should the brother /sister who had less to do with your aged parent benefit from it?