IHTM22001 | Interest in unadministered estates and QSR: introduction
From HM Revenue & Customs · Inheritance Tax Manual
Where a person is entitled to an interest in another person’s estate (IHTM04043) and they die before the administration of that earlier estate was completed you will need to calculate the value of the unadministered estate as part of the estate you are looking at now.
Where inheritance tax was payable in connection with the earlier death you may be able to allow some of that tax as a relief against the tax payable on the second estate. There is a similar relief on lifetime transfers (IHTM04051) on the termination prior to death of a qualifying interest in possession.