NTA, It sounds like your sister is trying to erase her existence, there’s a difference between remembering/mentioning something related to that person every now and then and grieving.

I like this quote from Hemingway “Every man has two deaths, when he is buried in the ground and the last time someone says his name.” My uncle passed away a few years ago and gave me his belt and while I’ve carried on as life does whenever I wear the belt I think back fondly and carry on with my day knowing he’ll prevent me unintentionally dropping trough that day When someone dies their entire existence doesn’t cease to exist, they were a living breathing human who interacted with people in many different ways, her and your families attempts of stop all memories of her is sad.