In my case, my brother was a newborn when I was three and a half. I was told he died during childbirth but I somehow knew that to be a lie. I lived with that knowledge and the unresolved grief for nearly twenty years until I discovered the truth of what had happened to him.
I hope you dont mind if I dont go into the details, Boots. But, suffice to say, siblings do grieve for their lost brothers and sisters and its very difficult if their absence is a family secret. The trauma of separation from significant others often results negative repercussions for the surviving children during their adult lives. Believe me, knowing is much better than not knowing. It brings with it sadness, a sense of closure and, eventually, an end to grief.