I never post, but wanted you to know that I feel so bad for you learning this information.
Maybe your mother didn’t understand the extent of the crimes he served prison time for ... or had no knowledge of his suspected connection to the murder of Judith or other victims. She only had his story to go on and believed it.
Guessing she didn’t marry him recently, so she was not afforded the ability to google him and read what you have no doubt read. All that said . . .
I understand how upsetting it must be that this person was in your family for any length of time. It has to be devastating to connect the dots and learn this someone you knew and that was intimately involved with your family, even your children, is really THAT person!
May not help, but I would think that if you found any evidence, even the slightest, among your mother’s belongings that belong to Cook (for DNA) or any clues to his activities or whereabouts before, during or after their marriage, you hang on to it. That are how cold cases and the unsolved get justice these days.
Thinking of you. Wishing you the best. And because I do, praying for peace.