If the case is as you have mentioned that your sister had been breastfed just one time by your maternal aunt, then the right Fatwa (legal opinion issued by a qualified Muslim scholar)
is that suckling only (Part No. 21; Page No. 35) renders marriage unlawful when it takes place five times or more during the first two years of lactation. Allah (Exalted be He) says:
The mothers shall give suck to their children for two whole years, (that is) for those (parents) who desire to complete the term of suckling
It is authentically reported that
`Aishah (may Allah be pleased with her) said that Allah's Messenger (peace be upon him) said:
One suckling or two do not make (marriage) unlawful.
Related by Imam Ahmad, Muslim, and the compilers of Sunan (Hadith compilations classified by jurisprudential themes).
It is related on the authority of
Um Al-Fadl (may Allah be pleased with both of them) that
A man asked the Prophet (peace be upon him) about the ruling of one suckling. The Prophet (peace be upon him) said: To suck once or twice or have one complete suckling or two do not make marriage unlawful.
According to another narration, she said:
A man came to Allah's Messenger (peace be upon him) when he was in my house and said: Allah's Messenger, I have a wife and I married another woman but my first wife claimed that she had breastfed my newly married wife once or twice. Thereupon, Allah's Messenger (peace be upon him) said: One suckling or two do not make marriage unlawful.
These two Hadiths are related by
It is also authentically reported that
`Aishah (may Allah be pleased with her) said: "It was revealed in the Qur'an that ten clear sucklings prohibit marriage and it was then abrogated by five clear sucklings. (Part No. 21; Page No. 36) Allah's Messenger (peace be upon him) died while the latter ruling in force.These Hadiths clearly negate that just one or two sucklings should render marriage unlawful. The last Hadith related by
`Aishah implies that giving less than five sucklings should not render marriage unlawful. This Hadith qualifies the general texts, from which some scholars inferred that suckling, whether little or much, renders marriage absolutely unlawful. These texts include Allah's saying:
Forbidden to you (for marriage) are: your mothers, your daughters
to His saying:
...your foster milk suckling sisters
They also qualify the authentic Hadith that the Prophet (peace be upon him) said:
Milk relationships are equal to blood relationships in rendering marriage unlawful i.e. whatever is prohibited due to blood relationships is also prohibited by milk relationships.
It is worth mentioning that if a baby sucks even little milk from the breast and then leaves it, this is considered one time of suckling. If he does it again and sucks even a little milk and then leaves it, this is considered another time of suckling, etc.May Allah grant us success. May Allah's Peace and Blessings be upon our Prophet Muhammad and upon his family and companions!