Still going with this character primer. Because it's my blog and I can.
His alignment is definitely Lawful Neutral. Although, I think, he desperately wants to be Lawful Good, his inability to speak up when it is necessary for fear of breaking ranks or the rules, he often ends up favouring action that is lawful over action that is right.
That is the primary difference between himself and his adopted brother, who will speak up for what's right even if it's against the rules or breaks all kinds of norms.
In the beginning, Wei Wuxian and Jiang Cheng have quite a good relationship, though not without it's sibling tensions. But, Jiang Cheng's temper and jealousy end up getting the better of him, and he and his once close adopted brother end up more or less estranged.
Part of Jiang Cheng's story is him coming to terms with the damage he's done, and forgiving both his brother and himself for the terrible things that happened to their beautiful family.
All in all, Jiang Cheng is not a bad person. What he is, is young and inexperienced, forced into a leadership role before he is ready, and trapped by all he thinks that is supposed to mean.
The elder sister of Jiang Cheng and adopted elder sister of Wei Wuxian, Jiang Yanli is an impossibly sweet, forbearing young woman, in whose position I'd probably end up going mad.
She mediates often between Wei Wuxian and Jiang Cheng, spoiling them both with her kindness and inability to get mad at either one of them.
In love with the heir of the Jin Clan (whom Wei Wuxian dislikes intensely, by the by), it takes a long, long time before her feelings are reciprocated, and even then, her happiness is extremely brief.
Her alignment, I feel, is Neutral Good. Ridiculously Good. She abides by the rules and doesn't step out of line herself, but she tolerates and even encourages Wei Wuxian's chaotic nature (mostly), and forgives so, so, so much.