Ashley's fixation on Andrew comes from the fact that, for her entire life, he's been the only constantly present person and source of affection she had. Due to being born with unacknowledged and unaddressed bipolar personality issues, she was tremendously difficult to deal with as a child and drove off everyone around her. Including, tragically, her own parents who were so depressed/negligent that they dropped her off on her brother to raise in their stead. No-one likes Ashley, no-one ever liked Ashley, not even her own parents, and she is desperately aware of that fact.
Enter Andy. Her brother, the one constant in her life. Except he's not there because he wants to be, not really (he actually is but his mental issues are a whole other thing). From Ashley's point of view he's only here because he's forced to be and has no other choice. Her massive inferiority complex won't allow her to think anything else, that if Andy had any other choice in the matter he would drop her immediately, and she needs him as her only source of human connection. This is where all of her schizo behaviour comes from: she's insecure and wants reassurance that she's valued and loved, and if/when she doesn't get it, she resorts to manipulation and emotional abuse to try and force it.
Other women are prime among her list of "other choices" for Andy so she's extremely sensitive to their presence, but she barely seems to actually understand the sexual component of a relationship. She knows the act intellectually, knows Andy desires it, but in terms of her actually having sex? Nothing. Barely any interest at all - the most she shows is a sort of transactional speculation if she thinks it'll get her something. She'll even accuse her parents of "cheating" on each other just for having friends outside their partner, perhaps as a result of her stunted emotional development.
It's actually Andrew who's driving almost all of the sexual aspect of their relationship. He is obsessed with her but he just can't admit it to himself, because he's caught up in his own lies about being a good person who's just powerless in the face of his yandere sister. Where Ashley's development into a good end relies on her accepting that Andrew does care about her & she doesn't need to control him, Andrew needs to acknowledge his own desires & that he isn't a helpless bystander in his own life - and if either sibling refuses to trust in the other, both get the bad "Decay" ending where they devolve into their worst aspects and kill each other. If they instead choose to embrace the positive aspects of their bond, trust and be honest with each other, they get the "Burial" ending where they bury their childhood traumas and toxicity.
Which is the best part: the incest and cannibalism is just the bait it uses to hook you in. What it really wants to talk about is morality, namely by which morals do you judge them, asking why do you hold those morals, and if your morals fail someone are they still beholden to live by them? By the time you actually get to the incest the game has spent so long slowly deconstructing your judgments of the siblings that you're left asking yourself if it's really worth continuing to live by what's clearly not working. If the siblings do the traditionally "moral" outcome in rejecting each other, the result is only more insanity and suffering. Meanwhile the traditionally "immoral" choice leads to psychological improvement and a healthier life for both. Is your reflexive disgust of their relationship really worth what it would entail?
I really loved Coffin of Andy and Leyley, if you can't tell, and the fact all of this shit goes completely over the heads of the people who bitch that it's just crazy sisterwife porn gets to me.