Is the son of her husband’s daughter regarded as a mahram for her?
Question

                Is it permissible for a woman who is over sixty to not wear hijab in front of the son of her husband’s daughter who is fifteen years old? He is like a grandson to her. Is it permissible for her to shake hands with him or kiss him? Her husband who is the grandfather of this boy has passed away.
Praise be to Allah.
    Firstly: The son who is asked about here is regarded as a 
    mahram for this woman, so she may shake hands with him and uncover in front 
    of him as she uncovers in front of her mahrams, and it makes no deference 
    that his grandfather is deceased. 

    The way in which he is her mahram is that she is the wife of 
    his grandfather on his mother’s side. Permanently forbidden in marriage for 
    a man are his father’s and his grandfather’s wife, no matter how far the 
    line of ascent reaches, whether the grandfather is on his father’s side or 
    his mother’s, because Allaah says (interpretation of the meaning): 

    “And marry not women whom your fathers married, except 
    what has already passed; indeed it was shameful and most hateful, and an 
    evil way”

    [al-Nisa’ 4:22]
It says in Zaad 
    al-Mustaqni’: The marriage contract makes the wife of the father and 
    every grandfather a mahram. 

    Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said in 
    his Sharh: The marriage contract is valid, and makes forbidden in 
    marriage the wife of his father or grandfather, no matter how far the line 
    of ascent reaches. Every woman who has been married by his father, even if 
    he divorced her, is permanently forbidden in marriage to him, and the same 
    applies to every woman who was married by his grandfather, whether on his 
    father’s side or his mother’s. The evidence for that is the verse in which 
    Allaah says (interpretation of the meaning): 

    “And marry not women whom your fathers married, except 
    what has already passed; indeed it was shameful and most hateful, and an 
    evil way”

    [al-Nisa’ 4:22]
If he marries her, that 
    is worse than zina, because Allaah says concerning zina (interpretation of 
    the meaning): 

    “And come not near to unlawful sex. Verily, it is a 
    Faahishah (i.e. anything that transgresses its limits: a great sin, and an 
    evil way (that leads one to hell unless Allaah Forgives him)”

    [al-Isra’ 17:32]

    But concerning this, Allaah says: “indeed it was shameful 
    and most hateful, and an evil way”. Marrying mahrams – Allaah forbid – 
    is worse than zina. Hence many of the scholars were of the view that the one 
    who commits zina with one of his mahrams, even if he is not married, is to 
    be killed. There is a hadeeth concerning that in al-Sunan. So the 
    wife of one's father or grandfather, not matter how far the line of ascent 
    reaches, is forbidden in marriage, whether on the father’s side or the 
    mother’s, and Allaah has not stipulated that the marriage should be 
    consummated etc, rather this ruling comes into effect by virtue of a valid 
    marriage contract. End quote from al-Sharh al-Mumti’, 5/198 

    This is a matter concerning which there is no difference of 
    opinion among the scholars. Ibn Qudaamah (may Allaah have mercy on him) 
    said, discussing the fact that a father’s wife is a mahram: It is the same 
    whether she is the wife of his father, or the wife of his paternal 
    grandfather, or the wife of his maternal grandfather, whether near or far.  
    There is no dispute concerning that among the scholars as far as we know, 
    praise be to Allaah. End quote from al-Mughni, 9/518, 524 

    Secondly: 

    With regard to kissing, she may kiss him so long as there is 
    no risk of fitnah, and it is better to kiss him on the forehead or head. 
Imam Ahmad (may Allaah 
    have mercy on him) was asked about a kissing a mahram man. He said: If he 
    has come from a journey and there is no fear (of fitnah). But it should 
    never be on the lips, only on the forehead or head. 

    Al-Adaab al-Shar’iyyah by Ibn 
    Muflih, 2/266 

    Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was 
    asked about kissing mahrams. He said: 

    If kissing mahrams is done with desire or one fears that 
    desire may be provoked, it is haraam without a doubt. If there is no fear of 
    fitnah then there is nothing wrong with kissing on the head or forehead. As 
    for kissing on the cheek or lips, that should be avoided, except in the case 
    of a father and daughter or mother and son, for example. It is more lenient 
    in this case because Abu Bakr (may Allaah be pleased with him) entered upon 
    ‘Aa’ishah (may Allaah be pleased with her) when she was sick and kissed her 
    on the cheek, and said, “How are you, O my daughter?” End quote, from 
    Fataawa ‘Ulama’ al-Balad al-Haraam, p. 691. 
And Allaah knows 
    best.