I know how it feels to loss someone so close and not understand why they died.  I lost my only sister last April and we still do not know what caused her death and to this day I don't accept that she has left us.  I am so sorry to read about you sister, and knowing that she was so young really touches.  All I can tell you that this pain will always in your heart and mind.  Like a friend told me once, we just learn how to live with the pain of losing someone, especially a sister.  I just hope God gives you and your family the strength to cope with this situation.  You will be kept in my prayers.  Take Care of you and your baby. :/