I have a personal headcanon that Kristoph started off as a normal, healthy child but when he was around 10 (Klavier would be around 2-3 at this time) he because increasingly sickly and weak and eventually bedridden. He could barely move and spent most of his time asleep in bed or studying whilst little Klavier got to play outside in the sun and seemingly got all of their parent's attention. Although Klavier could most often be found curled up against Kristoph as the elder read to him.
Even before his sickness, Kristoph's parents were quite protective and even more so now, yet they hardly seemed to bare down the same way on Klavier. Kristoph became paler, hair and eyes becoming lighter. He needed glasses. He was constantly thirsty (for tea). Because one/both of Kristoph's parents were actually poisoning him. In my eyes, it was his Mother and his Father and Klavier had no clue.
All those physical changes and his debilitation mirrors when Diego Armando was poisoned. Godot's were more drastic, as Dahlia gave enough to almost kill him off in one dose. However, Kristoph was sick for years, and so his aren't quite as bad.
He was given not nearly enough to kill, as this had gone on for years, but the Gavin Mother was so protective over her kind, smart, beloved firstborn that she wanted to keep him closed-off from the world and solely reliable on her for the rest of his life. She was a very obsessive, disturbed woman, but the Father was so enamoured with his wife that he never saw her that way. (Her reasoning for why they even had Klavier in the first place is so they could "have a child each".)
Then their parents both died in a car accident (the boys around 13 and 5) as per the most popular community headcanon, and Kristoph eventually gets better when doctors realise just what his mystery illness really was. As soon as he is well enough to leave the Gavin House and Germany (which takes another few years, at about 17-18 and 9-10), he takes Klavier with him to America while he studies law. Even now, Klavier is still scared that Kristoph could end up permanently in a bed or even dead from his "illness" returning.
Somewhere, deep down, Kristoph does know the truth about his Mother poisoning him, even though the doctors never told him the truth. And that's why it's his preferred method: it runs in the family.
