My twin Turkish angora kitties are so affectionate and nurturing to one another. I have never witnessed this level of closeness among any other pets that I have ever had so my guess is that it is the special twin dynamic.
When one twin is ill, the other comforts in the most compassionate and loving way.
I know that I would love to have been born a twin and I can imagine all the fun and innate bond that is only unique to us.