@trying2learn,
My best friend died on mother's day on 2009. I felt the same way for a long time after. But it does get better.
The odd thing about having someone so important die and leave you to deal with the loss. For a long time, I felt abandoned by her ... almost betrayed by her dying.
And out of the blue, every once and a while walking down the street of NYC and I'm still looking for her as if she's still around. A self delusion that almost as if she's packed up and moved to another neighborhood and that I'm expecting to see her doing chores or shopping or something.
Just the other week when I was walking to the library in midtown, a taxi pulled up to its stop and even if I'm not thinking of my friend that weird and truly unexpected thought of ... hey, she's just arrived in a taxi then a second later I realize that's literally impossible and I'm wondering where that stray unaccountable thought came from.
What I'm trying to say is that the pain of your sister will eventually lessen though I can't say exactly when but you will still remember her even when you're not directly thinking of your sister and Who knows what and when anything will cause you to think of her but you will be reminded of your sister at the oddest times and that's a good thing I suppose.