I think that was @r218 said is true but to add on I think her father was just a shitty person in general so he would take his anger out on her, if he truly loved her, he wouldn't even abuse her to begin with or get angry with her bc of something so small like her stutter. tbh I dont think its a big deal, everyone has troubles with something but if you work it out with them properly, they can resolve it. my sister had social anxiety and a stutter too but now she doesnt get as nervous being around ppl so she stutters less, and even on some days, she doesnt at all!
Spoiler
So riftan is often visiting croix castle as a representative of the remdragon knights. One early morning he stumbled upon maxi reciting poetry. This is when he realizes that she stutters and its not bc shes tense and nervous. He views her stutter as "nothing but an insignificant flaw". He follows by saying that, "he fully grasped how shameful it felt to have one’s weakness revealed to others". He warned everyone so that when she meets them this "flaw" isnt a flaw, if no one reacts or sees it as one. (●'◡'●)ノ
well maxi's father did marry her mother for political gain from the start so he never really loved any of his daughters,, he favored the other daughter because he benefits from her with her beauty. and i also think that in their times a stutter is very unusual and seen as a weakness due to "lack of confidence"? its very obvious maxi's father doesnt want anything "weak" to be related to his name,,,i really want riftan to sock him in the face fr

Sorry if i sound rude, but why didnt they react toward Maxi stuttering? I mean like her father made a big deal abt it, but riftan and his men as well as his staff was kind enough to not pay attention to it. Were they told beforehand? Or they simply don’t care? Im just curious