Corrected entry: Earlier in the series Slater said more than once that he was an only child. All the sudden he has a sister.
Corrected entry: Lisa, Zack & Screech all talk to JB as if they have known her since she was little. However, they have only known Slater for approximately four years, so them knowing her from a little girl, as implied, wouldn't be possible.
Correction: JB is only featured in one episode. The fact the kids knew her already (even if up to four years) doesn't mean she was known since a little girl...just that they all previously met her at some point.
Correction: Slater never said he was an only child in any episode. Only Zack and Screech have each confirmed they were an only child (Screech confirmed it in this episode). JB was just never mentioned before this episode. His mother was never seen in the whole series, but she still could have easily existed.