[A Couple Who Reached Old Age, Then Were Informed of a Breastfeeding Relationship Between Them]

Answer—and Allah is the One who grants success and support: If the couple attain a preponderant assumption (ẓann ghālib) of the truthfulness of the woman who informed them of the breastfeeding, then it is obligatory upon them to separate.
What is meant by the preponderant assumption is that which is above fifty percent. If what they have is fifty percent that her statement is true—or forty percent—then separation is preferable for them, but not obligatory. This is how the scholars of the madhhab have set it forth.
The proof for what we have mentioned is that a preponderant assumption takes the place of certain knowledge in many legal matters where certainty cannot be reached.
The proof that separation is not obligatory in the state of mere doubt or suspicion is that the default is the non-occurrence of breastfeeding, and one may not depart from this default without evidence; and what yields only doubt is not evidence. The wisdom—Allah knows best—may be the mutual cancellation of the two possibilities due to their equal weight, which leads to both of them falling away, like two opposing sets of testimony: if they are equal, they are both set aside.
As for the fact that separation is preferable in such a case, it is due to the element of precaution.
If it is said: It has been related, in meaning, that a man said to the Prophet (May Allah bless him and his family and grant them peace): “I wish to marry a woman, and a Black woman told me that she breastfed me and her; yet I think she is lying.” He (May Allah bless him and his family and grant them peace) said: “How will it be with him, once it has been said?”—this indicates something contrary to what the scholars of the madhhab state.
We say: That report concerns entering into a marriage, whereas what the scholars of the madhhab mention concerns exiting from a marriage. There is a difference between the two—ponder it.
If it is said: This answer does not stand, for the narration establishes that the man had already married; then the response is: The Prophet’s (May Allah bless him and his family and grant them peace) instruction to the man to separate from his wife was in the mode of recommendation and precaution; the proof is the following narration:
It is related in Sharḥ al-Aḥkām that a woman came to a man who had married a woman, claiming that she had breastfed them both. He then went to ʿAlī (peace be upon him) and asked him. He said: “She is your wife; no one renders her unlawful to you. Yet if you choose to be scrupulous, that is better.”
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1