I only have one suggestion.
You wrote a letter to all of us here....You should write a letter to your sister, even though she is gone.
You should write another letter to your niece and your parents also.
Pour your heart out to them all...Tell them how you feel.
Will it do any good? Maybe.... I think so. Plus there is something powerful about putting it all on paper.
Plus they will never be able to say. "They didn't know how you felt about your sister passing."
Namaste my friend