My sister boyfriend died how can i comfort her?

my 15 year old sisters bf just died 3 days ago in a car accident and i had to break the news to her, it was very heartbreaking to tell her the whole time she was screaming and crying saying "NO HES NOT DEAD HE PROMISED ME HE WOULDN'T LEAVE ME UR LYING IM GOING TO CALL HIM AND TELL HIM TODAY'SYS OUR 3 YEAANNIVERSARYRY!! HE CANT BE DEAD!" and alot more it was so sad i really felt bad......and like she said she called him and he didn't answer the 1,2,3,4,5,6,7,8,9,10 etc. time and then she came to me and just cried in my arms didn't know what to say to her but i knew she wanted me to comfort her i did the best i could but don't guess it worked because last night while she was sleeping i sat on her bed and she was wearin a tank and i saw the cuts she made and a year ago we lost our mother and grandfather in a fire so its been hard for us but how should i help her my dad is never around and i have a 12 year old sis too and im 18 im pretty much raising and taking care of both of them.....how can i help her i know she loved him