Dream of person, died, mother

I keep dreaming, almost every night, that I had another sibling that died and I had forgotten about. They are not always the same person, age or even gender. I just keep finding myself somewhere and I am really happy until somebody comes up to me and talks about the death of my sibling, which gives me this horrid feeling of guilt because I had forgotten all about there existence, then i spend the rest of the dream grieving for their loss. I even had to telephone my mother just to check that I hadn't been really selfish, and there really was another sibling. I have two younger brothers, my mother never had a miscarriage, although doctors did suspect my mum may have been carrying twins with my youngest brother, but that the other one died before the first scan as she had two blood types while she was pregnant with him, but he is 15 years old now so I hardly doubt that is what is causing these dreams. It is just really bothering me.