The ruling on a woman being in seclusion with one whose mahram status is doubtful

Question:
One of the female listeners, Umm Faisal, says: I have a brother who is twenty years old, and he was breastfed by my uncle’s wife. He eats and drinks with my uncle’s daughter, who was breastfed with him, and he takes her to school and is alone with her just as he would be with his sister. We do not know how many times he was breastfed by my uncle’s wife. The question is:
What is the ruling on her sitting with him and his being alone with her, when we do not know the number of breastfeedings as I mentioned before? May Allah reward you with good.

Answer:
He does not become her mahram or her brother unless the breastfeeding is established. It is necessary to establish that the breastfeeding was five times within the first two years, by the testimony of a trustworthy woman or a trustworthy man, or more than that.
As for if the number of breastfeedings is not known and there is no one to testify to the number of breastfeedings that are considered valid, which is five or more, then she is not his sister, and he is not permitted to be in seclusion with her, nor is he permitted to take her alone to school or anywhere else; because he is considered a non-mahram (foreigner) to her, unless it is established that he was breastfed by her mother five times or more, by the admission of her mother or by the testimony of a trustworthy person who knows that, or a trustworthy woman who knows that. Yes.