and that's exactly one of the many reasons why twins don't like being twins. i have seen many twins in my life and none of them enjoyed the fact that they were twins..though only in anime they portray that twins like to be together...
I've seen programs on tv about twins though where they are inseparable too - and say things like nobody could be closer or understand what it is like, or who have lost a twin and say every day feels like something is missing even decades later.. So I do think twins can be exceptionally close.
The issues I wondered about only apply to identical twins ofcourse. A regular twin will look no more like you than any normal sister. And Like Nez said, I think with identical twins, an effort should be maintained to dress and look different (In RL) Only in anime do they usually dress/look and act the same.
I love twincest stories myself, but think they work better when the twins are NOT identical, (like Kagami/Tsukasa etc) -
when they look the same I think it must be weird like kissing yourself lol (as someone said, it seems very narcissistic).