Is it sufficient to tell his fiancée of a fault or must he tell her family too?
Question

                I am 25 years old and my sperm count is low; it is between 1-5% lower than the norm. 

The doctors think that it would be difficult for my wife to get pregnant as a result, and it may be possible to use artificial means but the chances of success are also low. 

Now I want to propose marriage to a girl, and I have told her frankly about everything, and she has agreed and says that everything is in the hands of Allaah. 

My question is: It is essential to tell her family about this matter or not?.
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.