the only source we have for this presented for this was the Aruch hashulchan which was contested(due to the assertion(I dont have an aruch hashulchan in front of me) that it doesnt say you cant touch). The mishene berura in OC is probably discussing looking at an undressed girl.
So, what is the source for your statement? She may be no different, but why does that make them both forbidden. Maybe they are both permitted?
A neice is no different than any other strange girl is concerned, as far as the impermissibility to touch her.