Now when they remember my birthday they remember their grandmas death

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On my 19th birthday I woke up to a phone call from my sister. Her grandma that was only in her 60s died suddenly out of the blue from a stomach anurism. I just feel awful that it happened on my birthday so I completely ignored that it was my birthday and talked to my my sisters to make sure they were okay. (They are my half sisters from my dad) 
And I just feel terrible. Now every year on my birthday they will be mourning their grandma and its terrible 😭