Hanafi
The sources give no view for it here.
سبب القرابة بالرضاع
ما سبب ثبوت القرابة بالرضاع؟
Each school’s answer
In our wordsHanafi
Not stated in the sources
Maliki
Not stated in the sources
Shafi'i
Not stated in the sources
Hanbali
Not stated in the sources
The texts
رَابِعًا: الْقَرَابَةُ بِالرَّضَاعِ: ١٩ - لاَ خِلاَفَ بَيْنَ الْعُلَمَاءِ فِي أَنَّ سَبَبَ هَذِهِ الْقَرَابَةِ هُوَ حُصُول لَبَنِ الْمَرْأَةِ فِي جَوْفِ الطِّفْل، وَاخْتَلَفُوا بَعْدَ ذَلِكَ فِي الشُّرُوطِ الْمُعْتَبَرَةِ لِتَحَقُّقِ الرَّضَاعِ شَرْعًا، وَتَفْصِيل ذَلِكَ فِي مُصْطَلَحِ (رَضَاع ف ٧) .
«الموسوعة الفقهية الكويتية» (Kuwait) entry «قرابة» vol. 33 p. 74
From the schools’ own books
Maliki
The sources give no view for it here.
Shafi'i
The sources give no view for it here.
Hanbali
The sources give no view for it here.
What is «in our words» is our standard wording; the books’ texts stay as they are.