The only person who can sell your mothers house whilst your mother is still alive is your m other, and I assume she has nno intention of doing that, (and living on the street as a result) Of course if she did, and were to give the proceeds of sale to your sister, then that would be gift potnetially laible to IHT if youur mother were to die within 7 years, but as here total estae is below the IHT threshold then that tax will still not be payable. On the other hand if you are referring to your sister selling the house on your mother's death, then there will be no CGT to pay because of reason 2 in my first answer, unless the house is held as an investment for some while and appreciates in value, in which case a potnetial CGT liabililty will arise on any increase in value following your mother's death. Your sisters home will be her principal private residence and CGT exempt.