Are all the characters in the Buffyverse only children? The only ones I can think of that aren't are Angel (but he killed his siblings centuries ago) and Gunn (whose sister died in his first episode). And I guess Lorne also has brothers and sisters. And Buffy had Dawn from season 5 onwards.
But I don't think there was mention of any other characters having siblings. Xander and Willow and Buffy were all only children. Giles never mentioned any siblings, neither did Wesley, it didn't seem that Fred had any siblings, nor Cordy, nor Faith, nor Spike (although I suppose he could have). And Doyle wasn't around long enough for us to be sure if he had siblings or not. Oz might have had siblings. We knew almost nothing about his life. I guess Riley possibly had siblings, they just still lived on a farm in Iowa. Did any of the nerds have siblings? Wood didn't have any.
It seems that it's easier for the writers if the characters don't have families which are too extensive. But none of them (other than the ones I mentioned already) even referred to any possible siblings, so it's safe to assume they didn't have any.