That '70s Show Plot Holes

Plot Hole #1: In Season One during the episode "Eric's Burger Job", we get to see Donna's younger 14-year old sister Tina. However, after this episode Tina is never seen or heard from ever again. What happened to Donna's younger sister?

It is also mentioned that Donna has an older sister named Valerie, but the older sister Valerie character holds water because it was said that she was off at college.