I'm pretty sure that they're not supposed to be "Siamese twins", if that's what you mean. They're just "ordinary twins", wearing the same/attached outfit.
Twins are allowed to get that close. That's part of what makes them so adorable (unlike Siamese twins, who are stuck that way, which is sad).
(Unless you mean... y'know... their age. But I think the balance of age/sexiness just "feels right" for Nintendo's new approach.)