For a few years I sat on the pension committee at my old company. One of our tasks was to determine how a pension should be allocated in cases like this where there was no expression of wish. We had a set of guidelines to apply and these included considering all of the known family members.
Could be the case that you have had a group of pension trustees or committee members arrive at this decision.
There is probably an appeals process that she might access but if that happens they should also contact you for a statement.
I remember a few of these decisions where family splits were laid bare and we had to make some difficult decisions.