Peter and Dexter allude to something that often occurs in fiction, but is biologically nigh impossible in real life. (Identical twins are genetically identical, which includes the X- and Y-chromosomes.) Of course it can also occur that you have adult twins of which one is transgender. But in the case of Dexter and Sinistra, those two are merely siblings with a strong family resemblance. (With an age difference of one year, thus not even fraternal twins!)
Ah, I actually have a way to rationalize this: The boy was born first, and the parents gave him the relatively "normal" name Dexter. (This was of course way before people primarily associated this name with several TV show characters.) And when the girl was born, they decided to go with a theme naming and named her Sinistra! ;-)
And half identical twins in Autumn Bay? That sounds interesting!
Of COURSE Peter didn't come alone... we hope...
The "Half-Identical Twins" thing is something I've even got in the not-yet-shown parts of Autumn Bay...
And half identical twins in Autumn Bay? That sounds interesting!