Open Eyes, I can see that your family background is complex and that your therapist is working with you on understanding that.

It is complex and I don't pretend to have answers but there was one thing you wrote. It was about your father's belief truth was important but also that as children you were punished for lying.

Obviously we shouldn't judge by our knowledge now what was done in the past and 60 (?) years ago that was probably a typical method of reacting to children caught lying. It's just I am thinking that among other lessons learned in the home (who shouted loudest and scared others most got their own way) she also grew up with the reinforcement that lying must not be uncovered or it would be punished. It may have made her more determined to cover her tracks at all costs. Just as she has with you.

Modern parenting tends towards discussing with a child why they lied, explaining why it is wrong, but treating all parties with fairness and empathy. Adressing wrongs sure, but not punishing/shaming.

I am not criticising your parents as such because as you yourself say it was a very different time. Your sister is an adult now and responsible for her own behaviour but perhaps her experience as the older child who was punished is playing out now sadly.