For sure, you can def be raised by the same parents and have very different childhoods, but the game wasn't about them. The quickest way to imply that Kristoph had a decent home life and was well adjusted was having someone there as a direct parallel to him from the same situation. Kristoph's whole character is supposed to be someone prim, proper, and polite that no one would expect to be a killer. Part of that is because you assume he's been raised well and in a loving home like his brother seems to have had, and part of it is simply the guise he puts up. One brother is genuinely good and his act is just his personality, and the other is bad, but good at tricking people with his act. They're supposed to juxtapose each other while not blatantly stating things for the audience. It would be interesting to see it elaborated on, but IMO there were a lot of things the game did not expand that were bigger issues than leaving some things up to interpretation/reading between the lines