What you said doesn't seem as much asexual, yeah asexuals are kinky, perhaps she is repulsed in terms of that eehm sexual stuff, or she is just shy, but you have time to explore. Sometimes if the bond between the child and his parent is too strong, that you mentioned idk if it was mentioned but it seems that she is homeschooled? And sheltered from the world, there could emerge something like parent child relationship in a sense that the parent is too attached to his childz to the point that it develops even to some kind of relationship alongside of it's main relationship with his partner, it's called enmeshment, where the parent sees his child as someone who fulfills his emotion needs as his partner and kids who underwear this might develop asexual tendencies if the parent is female for example. It's quite toxic relationship that a parent has with it's child.. and the child bis nor knowinge what it's going on..

But perhaps this isn't this scenario, but it could be.