If a woman has breast feeded a girl once for a very little time and the same woman has a son, is it permissible that her son and that girl marries( she has breast feeded only once to that girl). They have already fixed the marriage.
Even a single suckling will establish mahram relationships because the Quran has mentioned it in general terms and not put any requirement of quantity or repetition:
حرمت عليكم ... وأمهاتكم اللاتي أرضعنكم وأخواتكم من الرضاعة
Prohibited to you are ... your [milk] mothers who nursed you, your sisters through nursing
As there is no Quranic verse or Mutawatir Sunnah that establishes such a requirement, so one should act on the apparent meaning of the verse.
This is the view according to the Maliki and Hanafi madhabs, and the view cited from a majority of the Sahabah, Tabi'un and Imams:
فروي عن عمر وعلي وابن عباس وابن عمر والحسن وسعيد بن المسيب وطاوس وإبراهيم والزهري والشعبي : " قليل الرضاع وكثيره يحرم في الحولين " وهو قول أبي حنيفة وأبي يوسف ومحمد وزفر ومالك والثوري والأوزاعي والليث
It is narrated from Umar, Ali, Ibn Abbas, Ibn Umar, Hassan, Saeed bin Musaib, Tawus, Ibrahim Nakhai, Zuhri and Sha'bi that: "Both a small amount or a large amount of suckling with establish prohibition, as long as it is within two years". And the same is the saying of Abu Hanifa, Abu Yusuf, Muhammad, Zafar, Malik, Thawri, Awzai and Laith
التحريم بالرضاع إنما يحصل إذا اتفق الإرضاع في الحولين ؛ كما تقدم في " البقرة " . ولا فرق بين قليل الرضاع وكثيره عندنا إذا وصل إلى الأمعاء ولو مصة واحدة
Suckling establishes prohibition when it is with the first two years of age, as mentioned in Surah al-Baqarah. Acccording to our view there is no difference between feeding a small quantity or a large quantity of milk. Even if it is a single sip, when it reaches the stomach Rada will be established.
The Shafis and Hanbalis have a different opinion. They have put a condition of at least five sucklings based on the narration from Aisha.