Well done @potatolord. As I suggested previously, your mum had a plan to make it even, it just appears your sister has rather tricked your mum. Personally, I would want a discussion with my sibling if they'd done this to the family, even if your intention is to then have nothing to do with her. I'd want my sibling to understand why. Maybe... maybe, it's an honest mistake of some kind. I would want to have the chat for sure.
This, on the other hand, I would suggest you should not do anymore. No more licking up backsides please, not even for a £300k house...your courage has given me the lick up the backside