Your sister is not sorry. And she's never going to change. If avoiding her improves the quality of your life, then do that.

It's too bad about your Mom, but that's the way the cookie crumbles. She has two daughters who don't like each other. No law says sisters have to like each other.

If you both show up at family functions, you can be civil. What involved the two teenage cousins should never have gone further. Your sister was wrong to come over to your house starting trouble. Don't expect the extended family to take sides. It's not their issue.

You don't have to "pretend" anything, but don't keep talking about it to other family members. Nothing they can do about it. They're not going to change your sister for you. You don't have to "get over it." Staying mindful of it may be how you prevent further trouble with your sister. Some people are good to stay away from.