Nope. Identical twins work perfectly.
Do I really need to point out what should be obvious as an internal contradiction in your post? The identical twins in the sinking boat are not the same person genetically. After admitting that fact, the rather weak attempt to handwave it away is not convincing.
So, here is the final exchange, "Hey, sis. Did you know that only 1 to 5 percent of identical twins are MZ? And you didn't make the cut -- I saved our separate placentas. So it's time for you to get wet!"
The salient point is that your "selfish gene" notion of morality does not require any charitable acts, and most certainly, not any heroic charitable acts.