The son of some people‘s sister is considered as belonging to the same people

Narrated Anas:

The Prophet sent for the Ansar (and when they came), he asked, 'Is there any stranger amongst you?" They said, "No except the son of our sister." Allah's Apostle ﷺ said, "The son of the sister of some people belongs to them."

حَدَّثَنَا سُلَيْمَانُ بْنُ حَرْبٍ، حَدَّثَنَا شُعْبَةُ، عَنْ قَتَادَةَ، عَنْ أَنَسٍ ـ رضى الله عنه ـ قَالَ دَعَا النَّبِيُّ ﷺ الأَنْصَارَ فَقَالَ " هَلْ فِيكُمْ أَحَدٌ مِنْ غَيْرِكُمْ ". قَالُوا لاَ، إِلاَّ ابْنُ أُخْتٍ لَنَا. فَقَالَ رَسُولُ اللَّهِ ﷺ " ابْنُ أُخْتِ الْقَوْمِ مِنْهُمْ ".