This is werid and it's happend a few times. In the episode "Blast from the Past" (the episode when Joey's old girl friend comes over to look for romance. And Kimmy is doing magic tricks and accidently hand cuffs Stephanie and DJ together and doesn't have a key), Kimmy said her brother had it.
In the first episode of Full House "Our very first show" DJ was in the garage and she said Kimmy had 3 siblings.
But then in the episode "Divorce Court" where Stephanie and DJ get in a big arugument, Kimmy said she was glad her parents stop having kids after her.
And in another episode it said she was the only child.

Was this mistake done on purpose? Does Kimmy really have any silbings?

I was just curious.

Kimmy has one brother, Garth, who she talked about a few times.
In the first episode, DJ says Kimmy has four sisters. Not true.
Only Garth is mentioned later on.

I always figured D.J was exaggerating, as that's soemthing a girl in her situation would do to convince her dad to let her have her own room, if she knows the odds are stacked against her