Maybe they don't want to admit it because they've been chasing after Mommy Dearest's approval their whole lives. Mommy's death would mean that she will never approve of them, never love them and never be a real mother. .
Piggybacking on my own post here --

And maybe this is why Mommy Dearest becomes Saint Mommy after she dies. The Scapegoat child of an N rearranges their memories so that Mommy DID approve of them, DID love them, and WAS a real mother to them, just like she was to the Golden Child.

And since Mommy was a saint, guess who was the one causing all the contention in the MIL/DIL relationship?