Who is uncle Fester?

Straight from one of the wiki's:

In the original sitcom of the 1960s, Fester is said to be Morticia's maternal uncle (his name rhymes with Morticia's mother Hester's), but from the 1970s onward, he is Gomez's brother. In all adaptations, he is either uncle or great-uncle to Wednesday and Pugsley.

In the 1960s television series, Fester (played by Jackie Coogan) is uncle to Morticia Addams. In one episode, he became perplexed when asked his last name, suggesting he has somehow forgotten it. In several episodes, Fester refers to the Addams lineage as if it were his own, possibly implying some degree of intertwined consanguinity in both their family lines, but the flashbacks in the episodes "Morticia's Romance, Parts 1 and 2," clearly establish him as Morticia's uncle, brother of her mother, Hester Frump, a.k.a. Granny Frump (Margaret Hamilton), whose maiden name was similarly unrevealed. In various episodes, he was a partner in typical sitcom schemes with Gomez, Morticia, or Grandmama Addams, indicating no real preference for any family member over others.

In the movie, The Addams Family, Fester (played by Christopher Lloyd) is the long-lost brother of Gomez Addams.

As for why, it's strictly about adaptation. Why are remakes different from the original movies? It's just a twist a writer thought could make their version more unique.