I'm the only child now because my brother passed away 2 years ago
Oh no I'm sorry to hear that!  Technically, though,  "only child" is generally applied only to those individuals who have never had siblings.
Oh, I didn't know that. 
No worries! Actually, throughout history, only children were pretty uncommon as many families had multiple children.
I wonder why? Maybe because they didn't have condoms
That's definitely possible! Though, the use of condoms foes back at least several centuries, so no excuses on that!