- To "other" it. Incest makes most people uncomfortable. And while there are a sizable number of only children, most people do have siblings of their own. Thinking about incest and siblings is weird. Making incest in a story about twins is a way to distance people from that, make it "other"—the majority of the population isn't a twin themself. This even justifies it in a way—they're not normal siblings, they're twins, and we all know twins are different and strange.
- Removing any age difference. Making them twins also serves the very practical and necessary function of removing the age difference and making them peers. And this is key for incest. Elder siblings are often patronizing or bossy at best, and mean at worst. Younger siblings are immature and childish at best, and annoying brats at worst. For siblings to fall for each other, they need to be peers.
- Making it non-serious and Played for Laughs. Twincest is taken seriously on occasion, but much more often it's funny or outlandish. Making incest bizarre and unrealistic is another way to lessen the taboos surrounding it.
Analysis / Twincest
And from an analytical standpoint, there are a few reasons an author might choose to make their incestuous characters twins: