Ascendants and descendants of the couple acting as witnesses to their marriage
Question

                Is it permissible for the woman’s maternal or paternal grandfather to act as a witness to the marriage contract?.
Praise be to Allah.In order for the marriage 
    contract to be valid, according to the majority of scholars it is essential 
    that it be witnessed by two Muslim witnesses of good character. It is 
    acceptable for the witness to be the woman’s grandfather on her mother’s or 
    father’s side, according to the more correct opinion. 
Some of the fuqaha’ did not 
    allow the ascendants or descendants of the couple or wali (guardian) to act 
    as witnesses. 
It says in Kashshaaf 
    al-Qinaa’ (5/66): The marriage contract cannot be valid if the witness 
    is biased, such as the sons of the couple or the sons of one of them and the 
    like, such as their fathers, or the son of one of them and the father of the 
    other, because of the risk of bias. End quote.  
i.e., he may be suspected 
    of being biased, because he is likely to testify in favour of his father or 
    son. 
It says in Sharh 
    al-Muntaha (2/648): The testimony of the wife’s father or grandfather 
    concerning [the marriage contract] is not valid, nor that of her son or 
    grandson. The same applies to the father, grandfather, son and grandson of 
    the husband, because of the risk of bias. The same applies to the father and 
    son of the wali. End quote.  
It says in al-Mawsoo’ah 
    al-Fiqhiyyah al-Kuwaitiyyah (41/300), discussing the conditions of the 
    two witnesses to marriage: The witnesses should not be sons of the couple. 
The Hanbalis stated – as 
    al-Mardaawi said: That it is stipulated that neither of the two witnesses to 
    marriage should be a son of one of the partners. So the marriage contract 
    cannot be witnessed by sons of the couple or by the son of one of them. 
The same conclusion may be 
    understood from the general views of the Hanafis and Maalikis, that the 
    testimony of a father for his son cannot be accepted, or of a son for his 
    father. 
The Shaafa’is have several 
    views, the soundest of which is that the marriage contract is valid (when 
    witnessed by the relatives discussed above). End quote.  
One report is narrated from 
    Imam Ahmad (may Allaah have mercy on him) which says that the witness of 
    ascendants and descendants is valid. This view was favoured by a number of 
    his companions. Al-Insaaf (8/105). 
Shaykh Ibn ‘Uthaymeen (may 
    Allaah have mercy on him) said: The second view concerning this issue is 
    that it is valid for the witnesses or one of them to be one of the 
    ascendants or descendants. This is undoubtedly the correct view, because the 
    testimony of ascendants and descendants is not allowed when it is testimony 
    in favour of a person, because of the risk of bias. But when it is testimony 
    for or against him, as is the case with a marriage contract, it is not 
    disallowed. 
The marriage contract in 
    fact is not exclusively about rights for the husband or wife, nor is it 
    exclusively about duties, rather it includes both rights and duties for the 
    one who enters into it. So the correct view is that the contract is valid. 
    This was also narrated from Ahmad (may Allaah have mercy on him) and was 
    favoured by many of our companions. End quote from al-Sharh al-Mumti’ 
    (12/99), 
And Allaah knows best.