Married with Children was never a serious show, but it did have a sad event behind the scenes in Season 6. Katey Sagal was expecting, and the producers decided to have Peg also be pregnant and to introduce a baby to the show. When Sagal sadly lost the baby, it was decided that it would be too hard on her to continue the new character. Peg’s pregnancy suddenly disappeared from the show with just the flimsy explanation that Al had dreamed it all. In the next season, the show tried again to introduce a new child, a cousin named Seven. When he proved unpopular with viewers, Seven also vanished with no word at all.
It’s not quite the same, but so far as I know, on That 70’s Show, Eric’s sister Laurie was played by Lisa Robin Kelly and then by Christina Moore, again in the sixth season, with the show pretending nothing was different.