I gotta admit, I have no idea who Alonon's sister is. I must of missed some content somewhere. What was her story?
She's Tyna's mom. She is mostly mentioned during the conversation with Alonon near the start of chapter 4 when the party inquires him about his almost "unperson-like" nature, he explained it wasn't entirely something that was done to him but rather that his asexuality was always a part of who he was and his work in the Tower merely made his body reflect his essence more closely, and also that he wasn't some kind of unique abnormality because his sister was the same.
The sequence of events then gets a bit iffy, at some point Alonon's sister resigned herself to a marriage for political reasons and at some point Alonon started to study magic and reality to try to understand his condition and find proof that sexuality wasn't a necessary part of normal people, which eventually led him to the Tower. But as it turns out in the TLS world, "lust" is one of the fundamental energies that composes everything, so in the end what he discovered is that his asexuality not only was abnormal but it made him incapable of appreciating a part of everything that existed.
It is unclear how much Alonon's sister was involved with his magical research, only that they had a close bond to each other, maybe she wasn't involved at all and was only part of Alonon's motivation, maybe she was with him every step of the way. Either way, at some point she supposedly died but we don't get many details about her passing.
It is a pretty far-out theory, but she would fit almost all the boxes for who the final mastermind could be (similar magic to Alonon, almost like an unperson, a woman) and she would have a pretty decent motivation to resent reality and want to change fate itself (which is more or less the thing that makes the final 3 powers be a "threat" to Ginasta and other tower mechanisms) since she was "forced" into marrying and bearing children even though that went against the essence of her being.