How about disappearing characters. Is there a term for them?

For example, On The 70's show's first episodes, Donna has a pesky younger sister. She quickly gets written out of the show and Donna becomes and only child.

I know I've seen this happen in other shows too.

I can't put my finger on it right now but there was something similar that happened with The Torklesons (spelling may be off). I think a little brother was eliminated when they moved - or something of the sort.