Praise be to Allah.
If the girl has agreed to get married, even though she knows about what you have mentioned about the lack of fertility and the possibility of not having children, and she is an adult of sound mind, then this is sufficient and it does not depend on her guardian knowing about it, because the right is hers.
The fuqaha’ have stated that if the husband or wife comes to know of a fault that may be grounds for annulment of the marriage, at the time of the marriage contract or afterwards, and accepts it, then the right to annul the marriage is waived.
Ibn Qudaamah (may Allaah have mercy on him)said: One of the conditions of this right to choose in the event of such faults being present is that he should not have known of them at the time the marriage contract was done and he should not have accepted them afterwards. If he knew of them at the time when the marriage contract was done or came to know of them afterwards and accepted them, then he does not have the option (of annulling the marriage). We do not know of any difference concerning that. End quote from al-Mughni (7/142).
In al-Mudawwanah (2/144) it says: I said: What do you think if she marries a man whose penis is cut off or who is a eunuch and she is aware of that? He said: She does not have the option (of annulling the marriage). Maalik said likewise. He said: Maalik said: If she marries a eunuch and she did not know about that, she has the option (of annulling) when she finds out. The view of Maalik is that if she knew (beforehand) then she does not have that option. End quote.
It says in Kashshaaf al-Qinaa’ (5/111): If one of the spouses who has no fault knows of a fault in the other at the time of the marriage contract, he or she does not have the option (of annulling the marriage), or if he or she came to know of the fault after the marriage contract but accepted it, he or she does not have the option (of annulling the marriage). It says in al-Mubdi’: And there is no difference of opinion that we know of. End quote.
Al-Sarkhasi al-Hanafi said: If she marries one of these [i.e., a man whose penis has been cut off, or a eunuch or an impotent man] and she knows his situation, then she does not have the option (of annulling the marriage), because she approved of it when she went ahead with the marriage contract although she knew his situation. And if she accepted it after the marriage contract by saying, I accept it, then the option is waived in her case. End quote from al-Mabsoot (5/104).
See also: al-Mawsoo’ah al-Fihqyyah (29/69).
It is well known that low fertility is far less serious than these faults that have been mentioned by the scholars.
The apparent meaning of the scholars’ words in this matter is that it is sufficient for the woman to know of the fault, and it is not essential to tell her family.
The issue of having children is not something that you should worry about. How many men have been told what you were told, then Allaah blessed them with offspring. The matter is up to Him, may He be glorified, and bounty is in His hand. You should apply whatever measures you can by seeking medical treatment, and ask Allaah of His bounty.
We should also point out that the fiancée is a stranger (non-mahram) to her fiancé and it is not permissible for him to be alone with her or to touch her, so any discussion about the marriage should be with her wali (guardian).
And Allaah knows best.