Paying off parents' mortgage in exchange for a share in their property

bolwin1

We paid of our in-laws mortgage about 12 years ago as they were struggling to make ends meet & my Father In Law (FIL) was ill. It was around £30k. They updated their wills to reflect this. We didn't put a charge put on the house, although I appreciate if they both had gone into care this would have been an issue. When my FIL passed away a couple of years later, my MIL  downsized & repaid us with the equity that was released.

xylophone

I see what you mean - nevertheless, your parents (and you) chose to help your sister, presumably without the expectation of return (other than that of paying the mortgage as  previously stated).
None of you have any right to expect that they should leave you the property in their wills, although of course they might choose to leave their interests to you and the other siblings to "even up" the situation.
Should both need  means tested care before death, then their interest in the property would almost certainly be taken into account.
You could protect any loan you made (and rolled up interest/share of proceeds on sale) through a  charge and legal agreement as outlined above as this would be a secured debt and would have to be paid on sale before any use of the money for care.