Lookalike Lovers

Alice sure looks like her mother quite well. You could say that she looks like a younger version of her mother. Later on, we see her father, who also looks like her. Wait, wouldn't that make the mother and father look suspiciously similar too? And that it does, when we see the whole family together!

The same thing can be said about couples. The two characters apparently are not related, yet somehow you could tell they would click because they look similar to each other.

There are many reasons why this happen. For parents, it could be explained that the son or daughter is conceived by the show first before creating the parents of the character. It could also mean a reflection on real life, as people are attracted to someone that looks like them.

Usually the blood relationship of the couple is stated or implied to be unrelated early on to avoid questionable stuff.

See also Birds of a Feather, which goes beyond the characters' looks. Related to Model Couple, especially when used for advertising.

Examples of Lookalike Lovers include:

Anime and Manga

One Piece has Igaram and his wife Terracotta, to the point some of the Straw Hats accused Igaram of being into drag. Zoro lampshades this.

Zoro: A couple can only look so much alike!

Possibly the reason Kuchiki Rukia and her brother Byakuya somewhat resemble each other, despite not being blood-related—she's his deceased wife's sister.

Film

One of the love interests in Le Samourai was cast because the actress looked like she could be Alain Delon's sister and the director wanted the audience to feel that awkwardness between them. What makes this particularly weird is that said actress was Alain Delon's wife.
Neo and Trinity in The Matrix trilogy look very much alike, both with pale, androgynous faces and short black hair.

Literature

Completely inverted in one Agatha Christie novel The Moving Finger: When every villager starts receiving hate mail, the main character (who lives with his sister) is accused of being husband and wife masquerading as brother and sister. They don't look anything like each other (as each takes after one very different parent), hence the accusation.
Inverted in A Song Of Ice And Fire where Cersei and Jaime Lannister are twins, but most people don't realize they are also lovers, and that King Joffrey is born of incest.
The Van der Veghels in Ngaio Marsh's When In Rome look suspiciously similar. Alleyn suspects, but cannot prove, they are in fact half-siblings. What's more, he suspects blackmail on this point prompted the husband to commit murder.
Sarah and Silas Heap in Septimus Heap both share Green Eyes and curly blonde hair with their children.
Lucius and Narcissa Malfoy in Harry Potter; especially striking since every other Black is a Eerie Pale-Skinned Brunette, but Narcissa somehow dodged that bullet.
The light novel version of Durarara!!!! have this with Ruri and Kasuka.

Live Action Television

An example of a Deconstruction that isn't grim or dark occurs on Thirty Rock, where Jenna Maroney begins a long-term relationship with a Drag Queen who works as a Jenna Maroney impersonator because she's so vain that she wants to make out with "herself".
In one episode of Seinfeld Jerry starts dating a woman who looks exactly like him (Janeane Garofalo).

Web Original

Twisted around a little by The Nostalgia Critic and The Nostalgia Chick who have a similar aesthetic (pink pouty lips, large green eyes, Hollywood Pudgy, pale, dark hair, often wear glasses). She won a contest created by him to be his Distaff Counterpart, and they might not be a standard couple, but they own a complicated sexual history.

Web Comics

The Winston Rowntree comic 4 types of husbands and wives has this as one category "the you with longer hair".

Western Animation

The Simpsons:
- Milhouse's parents look so similar to each other.
- Same case with Ralph Wiggum's parents, Chief Wiggum and his wife (albeit not to the extent like the Van Houtens).
Ralph the security guard and his wife in Animaniacs. Their son, who looks nothing like them, desperately hopes that he is adopted.
Family Guy Mort's wife Muriel, who looks just like Mort if he was in drag.

Real Life

It's actually proven that people are attracted to someone who looks vaguely similar to them, be it physically or mentally.