However, does survivorship trump intestacy rules ?
Yes, survivorship "trumps" the intestacy rules. However, it is crucial to appreciate the difference between legal and beneficial ownership.
X and Y may hold the legal title to a bank account ie each name is on the account. Technically, X and Y are bare trustees who hold the beneficial interest in the account for X alone as it is X who provided monies for the account and who withdrew monies as and when. Y holds no beneficial interest in the account.
On X's death, under the survivorship rule Y acquires sole legal
title. However, the beneficial
ownership was not held jointly as all the monies belonged to X. As a consequence, the beneficial interest in the account does not
pass by survivorship to Y. X is able under X's will to leave the monies in the account to whoever X wants; if X left no will then X's beneficial interest passes under the intestacy rules.
If the above reflects the facts then the person(s) who took out a grant of letters of administration following the death (as there was no will) is/are responsible for administering the deceased's estate. This should have included an investigation as to the precise relationship and nature of the jointly held bank account in order to determine whether the sister should have acquired the monies qua survivorship or not. Assuming that she should not have acquired the monies then the beneficiary(ies)who should have inherited can require the person(s) who took out the grant to make good their loss.