Lucy is afraid of anyone new entering the house, picks at her food, and is considerably smaller. Lucy also loves to chew up cardboard boxes. Also they both chew their back claws. Maybe you just need to change their names then get a couple of boy cats named Ricky and Fred. But they have less need for social interaction than humans do.
Some cats are even more the loner type than others.
Your girl Lucy is shy. She lives a life of stress with her sister Ethel. She acts that out on the holidays by unwrapping your presents. Cats have a unique set of nerve responses around their mouths that make them highly adapted predators. They are custom made to attack their food.