It’s been implied that Sour Cream and Onion are Half-brothers



Yellowtail, Sour Cream, and Onion are all seen together in The Return, implying they are a family unit of some fashion.



Vidalia is a type of onion. Vidalia is likely Sour Cream and Onion’s mom, completing a terrible trifecta of names.



Sour cream talked about his “weird step dad,” which likely refers to Yellowtail, meaning Sour Cream’s genetic father is unaccounted for.



The timeline is a little hazy, but if Sour Cream was conceived when Marty was in beach city, the difference between Steven and Sour Cream’s age would be the amount of time Greg and Rose dated before deciding to have a child, Which seems like it checks out.



Marty is Sour Cream’s dad.



Just look at these pasty human beings