When someone dies on your birthday

My sisters have a different mom and their grandma died on my 19th birthday.

When her death date and my birthday comes around, one of my sisters starts to feel it. Her gma was her very best friend and more like a mom to her. But I think she tries to not say anything or express herself because it's a day of celebration for me. And I don't know how to get it across, that she can say and express herself in any way she wants to, celebrate my birthday with me and also mourn for her grandma. I don't want to say something that's off putting or rude. How do you suppose I say that? Remembering her grandmas life will not put any kind of damper on my birthday. Both of those things have plenty of room to be recognized in one day. Make sense? It will be my 21st birthday.