七兄弟之親 Sworn Brothers Of Old
行者道：「尊府牛魔王，當初曾與老孫結義，乃七兄弟之親。今聞公主是牛大哥令正，安得不以嫂嫂稱之？」羅剎道：「你這潑猴！既有兄弟之親，如何坑陷我子？」行者道：「尊府牛魔王，当初曾与老孙结义，乃七兄弟之亲。今闻公主是牛大哥令正，安得不以嫂嫂称之？」罗刹道：「你这泼猴！既有兄弟之亲，如何坑陷我子？」
逐句解 Line by line
Pilgrim said, 'The Bull Demon King of your honoured house swore brotherhood with old Sun in years gone by — we were seven brothers by that bond.'
當 is the positional dāng of 當初, 'at the outset'. The seven sworn brothers of the demon kings belong to chapter three, five hundred years before the pilgrimage began; Pilgrim is producing a very old credential.
'And now I hear that the Princess is Elder Brother Bull's lawful wife. How should I not address you as sister-in-law?'
令正 is the polite term for another man's principal wife, matching the 令郎 he will use of her son. 安得不 'how could one not' — the argument is presented as a point of etiquette rather than of affection.
The Rākṣasī said, 'You wretched ape! If you were bound to him as a brother, how came you to entrap my son?'
She accepts the genealogy and turns it into the charge: the brotherhood makes the injury worse, not better. 我子 takes 子 as the full-toned noun zǐ, 'my son', not the toneless suffix. 坑陷 'to trap in a pit'.
Pilgrim's answer is a formal deduction. The Bull Demon King swore brotherhood with him; she is the Bull Demon King's principal wife; therefore she is his sister-in-law, and etiquette requires the address. He uses 令正, the polite term for another man's wife, and frames the whole thing as a question of what one is obliged to call people.
The bond he is citing is five hundred years old and belongs to chapter three, when the seven demon kings swore brotherhood and the Monkey King was one of them. That is before the mountain, before the sutra-quest, before he had a master. He is reaching back past his entire reformation to find something this household will recognise.
Her response takes the genealogy and turns it round. If you were his brother, how came you to entrap my son? The kinship does not excuse the injury; it aggravates it. She has accepted his premise and used it against him in twelve graphs, which is the first sign that the glib ape is going to have to work.