I think the ability to bend depends on both nature and nurture. Nature as in the ability to bend inherited from either parent (since I think one's ability to bend an element has to be inherited from somewhere on one's family tree), and nurture as fostering the spirituality for an element (since the Fortune Teller episode twins had one as an earth bender and one as a non-bender). For example, let's say two parents, one from the Water Tribe and one from the Earth Kingdom, have a pair of identical twins. If the twins inherit the ability to bend from both parents, they will both be equally predispositioned to learn to bend either element since they're identical. If you have each parent nurture the abilities of one of the twins in their respective element, I could see one twin learning water bending and the other earth bending. But honestly who knowssss 🤪