why is the uncle on the yang family side's daughter cousin but his 3rd uncle on the chu family side's daughter his younger sister? why does it seem like he isn't able to marry his younger sister but he was able to be be engaged to his cousin? i mean looking at the blood relations they both have the same status being related from chu grandfather bloodline and yang grandfather separately?
The sister\cousin bit I would assume it is because they live in Patriarchal clans so the Chu's are raised as one family so they use sister while the Yang's are classed as a completely different family so is referred to as a cousin because of the blood links. Could also just be being used as a sign of affection as the Chinese are always calling close friends brother or sister. OR https://en.m.wikibooks.org/wiki/Chinese_(Mandarin)/Family
Understanding ancient human relationships is quite tough. Just know that he will pork the one who gets the most points.