You know how Jaime and Cersei are fraternal (dizygotic) twins? And that's a real thing, that fraternal (dizygotic) twins can occur multiple times in a bloodline. Because genetically women can be predisposed to release more than one egg, and therefore have twins more often.
Except the only other time we know about twins in House Lannister is Jason and Tyland who are identical (monozygotic) twins and that's totally a different thing that is just a completely random process that happens when an egg splits into two for some reason?
And then next pair of twins we hear about are bastard twins Robert happens to have at Casterly Rock, which now implies that Casterly Rock has some kind of twin magic that isn't a genetic predisposition or explainable phenomenom in any way?
Oh and also, Tyrion might be a Aerys-Tywin chimera who once had a twin but I don't know if I believe that theory. (The only evidence is the mismatched eyes.) But it would make sense if Casterly Rock was like, "Damnit, Johanna, get another baby in that womb!"
Anyway! Am I the only one who thinks about this? Why does Casterly Rock want multiple babies in one womb? What's it got against single birth? I don't get it.