Millhouse Parents 

When a romantically-linked couple could easily pass for brother and sister.

An individual who chooses to date someone who shares their physical characters is worthy of ridicule as they are presumably guilty of narcissism (at best) or incest (at worst).

Can work as well for homosexual couples who bear a familial resemblance, prompting assumptions that they are brother and brother, or sister and sister.

Origin: From the Simpsons, Bart's long-suffering sidekick Millhouse Van Houten has similar blue hair, thick eye brows, and large glasses as both his mother and father. Additionally, it is possible given Millhouse's poor judgement that his parents do in fact share the same genealogy.

From "The Simpsons: Season 22, episode 3 entitled: MoneyBart (2010)
Bart has provoked his noted-feminist sister Lisa with dismissive terms such as "Honey Pie" and "Tootsie Pop". Prompting Nelson to interject with the following:

Nelson Muntz: Hey, get a room, you two.
Lisa Simpson: We're brother and sister.
Milhouse Van Houten: So are my parents ... I think.

Concerned friend #1: Do you think Matt's girlfriend kind of looks the same as him? I mean, they're the same height, same hair color, they even laugh at the same jokes...

Concerned friend #2: Oh yeah! Total Millhouse Parents.

Millhouse Parents by Piperdad June 23, 2012

Milhouse parents

when someone's parents look like they could be brother and sister, or very closely related

Wendy's mom and dad both have extremely similar noses, head shapes, eyes, smiles and hair color. They look so much alike that people often confuse them for being siblings. They're not siblings, they're just Milhouse parents.

Milhouse parents by Jane56 July 20, 2014