Bevendre

“You can’t ship Marble Pie and Big Mac!  They’re related!”

A) People can ship who they like, it’s shipping not the choosing stone.
B) Being related has never stopped other ships in many fandoms
C) They state in the show that they’re not even certain the two families are related, and the stated relationship in Pinkie Apple Pie would make the two more distantly related than the state of Alabama.