If the surviving sibling, say maybe in their eighties, has no other assets other than the house, is facing a bill of £80,000, the £80,000 has to be paid and they would likely have to sell the house to pay the £80,000.
Agreed they'd still have £445,000 remaining to buy a new house. Not bad at all, but a huge upheaval. All I suggested was a tweak to pay the £80,000 on their death rather than within six months of the first sibling's death.
Seems cruel when a tweak is all that is needed. Same tax will be paid, just later. I never claimed they will be practically homeless .