It's not ok and you should tell your sister that you need to talk about your mom as well. Your sister needs to realize that it is all part of the coping process for you as well.
She needs to offload but can't deal with you doing the same.... is she younger than you? Mind you, my older sister was like that. Different people cope with death in different ways. Talk to someone who'll listen.
She's too immature to deal with your feelings on this. Just talk to a friend who'll listen or someone you're close to. She needs to do it in her way and you'll find yours.
It's called emotional issues..... She might not be able to deal with it when you bring the subject up....
Everyone deals with tragedies differently.. One needs to speak it but doesn't wanna hear it.. But if you need to speak it too.. Let her know, otherwise.. let her be..