It's an in-joke, he's an only child. The joke is that his "brother and [his] sister" can't "speak to [him]" because they don't exist, so obviously he can't blame them.
But then the title reveals the depth of the song, in that he always resented being an only child because he never learnt the basic social skills that other kids developed through interaction with their siblings, like how "to share" toys and the like.
It's an in-joke, he's an only child. The joke is that his "brother and [his] sister" can't "speak to [him]" because they don't exist, so obviously he can't blame them.
But then the title reveals the depth of the song, in that he always resented being an only child because he never learnt the basic social skills that other kids developed through interaction with their siblings, like how "to share" toys and the like.