I don't think the brother even cares his little brother got hurt, he just cares that Abel is the one that Lilith likes, so now he is trying to show off to her and he is just gonna get his ass whooped.
if we're talking about the state of society at the time then it reminds me of europe around the time of gentry in which case it wasn't uncommon for people that young to be in relationships with anyone. There were many nobles that took young girls and many that took older women. I honestly think its just another one of those things when basing a world around such a time. All in all however, its a manga and the author does what the authoer wants.
@givemersspls note what penmore said, and also note that lilith is a demon (and like an elf) so her being 200 could be about age 10-17 depending on their longevity ...
also in medieval times kids would be born already engaged. if its an important political marriage then your fiance may not be born yet and you are already in your 20s and expected to create an heir as soon as possible (1st bleeding was age of marriage back then)
@givemersspls Adding on to genoshock and penmore, during the medieval periods noblemen were practically dictators, and life expectancy was shockingly low: barely over 30. It was normal for children as young as 14 to consider having their own, and women, young or old, the noblemen fancied were often taken by force or for money. If this were the actual medieval periods she might even be executed or kidnapped.