[Pilsner]

Its akin to natural transplantation. If you recieved a heart from a doner would you be your doner or would it simply be their heart in your body pumping your blood?

The incorporation of the twin's DNA happens when the embryo is BB size, or smaller. Not a heart, but cells that develop into a heart later.

[Bayard]

Replace heart in my example with, cells and blood pumping to cellular functions.

The point being, that its just another form of transplantation, regardless of the time or stage of development.