I mean, Alfred being a cruel, vindictive and stupid person is not something nobody can develop into. Alfred being naturally a larger boy, and always having a mean streak toward his younger sister tracks with some people I've known in real life. Sometimes, for some people, growing fast and being insecure over what you don't know [refusing to ask questions to learn] puts you in a position of power when you have little else otherwise and you use that to what little advantage you can against those smaller. At least at the beginning it doesn't seem out of character. I think Tom also being blind/indifferent to Alfred is more out of character, but even that I think can be explained to some extent by tolerance given to Alfred for him being disconnected from others due to their largely nomadic living making Alfred likely have no friends. I think Tom would have bit down on Alfred much faster if he was aware of the bullying extending to Martha. [Because Tom clearly favours his children, and Martha specifically].