Marge is actually the real mother

Laughed too hard at poor Milhouse

His parents are actually cousins from the same town of Shelbyville.

Well they do look alike.

WAIT A SEC IF MILHOUSE'S PARENTS ARE SIBLINGS THAT MEANS MILHOUSE IS A BASTARD WHO CAN POSSIBLY BECOME AN EVIL KING WHO SITS ON A THRONE MADE OF SWORDS AND ALL THE PEOPLE OF SPRINGFIELD WOULD BE LIKE "TOTALLY CALLED IT" OMG YES LETS GO BURN DOWN THE POST OFFICE