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)
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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.