(((Silversparrow)))
I don't know what to say. I cry just reading your story.
Don't feel responsible, keep those memories close to you. Tell your little sister all about them as much as possible it will be good for the both of you, even if you've repeated the same story a thousand times, keep on telling it.
Grieving is a necessary process and a long one at that. It's been 6 years since my Dad passed and I still cry for him once in a while. I cried in the fetal position for about a week after he passed, but I had to move on, dust myself off and go back to my life. Just as you do, it's hard. Anniversaries, holidays, birthday any special day, are especially hard. Still talk to them. Keep making them proud and take comfort in the fact that they are watching you, cheering you on.
Take care of yourself.