our mc displayed favorable opinions, treating his companions fairly and owns their resppect while being weak. he also has a bag that could contains any amount of items and he might be able to trade with other humans easily. so i guess, little sister strifes to make her village benefit from being married to such a person. it might also help fortify her big sisters situation, since she recently lost a group of other half-elves while she was out. so she might just be pretty calculative.
or she just likes to have fun with the human that appeared