As per title - is there such a word?
Either a noun:
John is an ?????? (his only brother died when he was 3)
or verb (or past tense as an adjective):
John was ????ed today (his only sister died today)
English Language & Usage Stack Exchange is a question and answer site for linguists, etymologists, and serious English language enthusiasts. It only takes a minute to sign up.Sign up to join this community
They are often mentioned as "bereaved sibling".
For example, in this book:
Sibling Grief: Healing After the Death of a Sister Or Brother - By P. Gill White
bereaved adj. Suffering the loss of a loved one: the bereaved family.
Someone with no siblings is called an only child. I have heard an elderly woman say, when her last sibling had died, "Now I have become an only child."