The episode "David, Goliath and a Yoo-hoo from the Back" (s2e14) really lines up with Sheldon's and Mary's personalities in tbbt. I will focus only on Mary:
In tbbt Mary told Penny "when I was your age you can have me with a car ride and a bottle of strawberry wine". After seeing the Mary from YS It's sounded really out of character, not to mention that she probably had Georgie at that point. Hearing that Mary was pretty wild as a teen and became really religious just AFTER the twin's birth makes so much sense. In a wider view It shows that Mary's way of dealing with crisis is to become more religious, which would explain perfectly why she became even more fanatic after George's death.
