The Prophet ﷺ said, "The son of a people's sister is one of them," or "is from among them."
حدثنا أبو قطن حدثنا شعبة عن معاوية بن قرة
عن النبي ﷺ قال ابن أخت القوم منهم أو من أنفسهم
Its isnad is sahih, and it has already been mentioned at 12793.