Porky Pig and Petunia Pig
The entire Disney crew (Mickey, Minnie, Donald, Daisy et al.) but most notably supporting characters Clarabelle Cow and Horace Horsecollar, who look related despite the fact that they're different species
Pac-Man and Ms. Pac-Man
And Toad and Toadette
Just to name a few.
As you can see, animators have been creating love interests for pre-existing characters and routinely making these sweethearts look like they share way too much DNA to be dating — or worse, mating. They really do look like brother and sister, all of these folks. It’s interesting that cartoonists would want to make these characters — most of them intended for young audiences — look like they’re committing incest. However, it’s more probable that the resemblance stems from (a) the animators’ lazy reluctance to create an entirely new model for a second-tier character and (b) the fact that having the characters look alike helps to tie them together. You see Daisy Duck waltz onto the scene, and even if you’ve never seen a Disney cartoon before, you know straight away that she’s involved with Donald in some way.
It’s like the Skeeter Syndome. As the people who made “Muppet Babies” clearly realized, a dearth of female characters can easily be remedied by making a female twist on a male character. That’s how the show got Skeeter, the girl counterpart to Scooter, who had existed in the regular, non-animated “Muppet Show” for years. (Granted, Skeeter was Scooter’s twin, not his love interest, but the idea still holds water.) In the end, any given group of characters gets to even out the gender ratio, but at the expense of looking like a bunch of inbreeders. And it’s been happening, apparently, since cartoon characters have been around.
An interesting note, however, is Kirk and Luann Van Houten. Since they’re on a show that seems to be conscious of trends on TV shows — especially animated shows — they’re resemblance could be intentional. How else could somebody explain why Milhouse is the way he is? What’s more, “The Simpsons” has featured both Kirk’s dad and Luann’s mom, who are on opposite sides of the family but nonetheless bear the resemblance. The twist comes in that the Van Houtens have been divorced for nearly as long as the show has featured them as a married couple. This, to me, is an interesting twist on the whole look-alike couple, and the kind of twist on a standard formula that once made “The Simpsons” a good show. The ones who look like they must belong together actually don’t and now hate each other.
And as for the Goldmans from “Family Guy,” I can only guess that they — being the awkward, bespectacled parents of an equally awkward, bespectacled son — are a Van Houten family knock-off, at least in a broad sense. But hey — “Family Guy” has to owe something to “The Simpsons,” right?