Ruling on shaking hands with and kissing mahrams
Question

                What is the ruling on greeting mahrams by kissing and shaking hands? If that is permissible, then who are the relatives who are mahrams? Does the ruling on shaking hands and kissing include mahrams through breastfeeding?
Praise be to Allah.
    There is nothing 
    wrong with a man greeting his mahrams, or with a woman greeting her mahrams, 
    by shaking hands or by kissing. There is nothing wrong with that. The 
    mahrams are the ones who are mentioned in the verse in which Allah, may He 
    be glorified and exalted, says (interpretation of the meaning): 
    

    “…and 
    not to reveal their adornment except to their husbands, their fathers, their 
    husbands fathers, their sons, their husbands sons, their brothers or their 
    brothers sons, or their sisters sons, or their (Muslim) women (i.e. their 
    sisters in Islam), or the (female) slaves whom their right hands possess...”

    [an-Noor 24:31].

    So mahrams include 
    maternal uncles, paternal uncles and others. 

    The following are 
    the mahrams: her father, her grandfathers, her mother’s father and 
    grandfathers on the mother’s side, her sons, the sons of her daughters and 
    the sons of her sons. The woman’s brothers and the sons of her brothers are 
    also her mahrams, as are her maternal uncles and paternal uncles; all of 
    them are her mahrams. The same applies to her husband’s father, her 
    husband’s grandfather, her husband’s son, his son’s sons and the son of her 
    husband’s daughters; all of them are mahrams to her. 

    There is nothing 
    wrong with a man kissing his mahram - his paternal aunt, his maternal aunt, 
    his mother, his grandmother and his sister. There is nothing wrong with him 
    kissing them, but it is preferable for him to kiss her on the head, 
    especially if she is an adult, or on the nose or on the cheek. The majority 
    of scholars regard it as makrooh to kiss on the mouth except the husband; it 
    is better for that to be with the husband, not with mahrams. In the case of 
    mahrams, kissing should be done on the head, the nose or the cheek. This is 
    what is preferred and is appropriate. 

    It is all the same 
    whether the mahrams are mahram by blood or through breastfeeding. 

    The mahrams 
    through breastfeeding are the woman’s father through breastfeeding, her 
    paternal uncle through breastfeeding, her maternal uncle through 
    breastfeeding, her husband’s son through breastfeeding, her husband’s father 
    through breastfeeding. They are like blood relatives because the Prophet 
    (blessings and peace of Allah be upon him) said: “What becomes mahram 
    (forbidden for marriage) through breastfeeding is that which becomes mahram 
    through blood ties.” This is what he (blessings and peace of Allah be upon 
    him) said. So blood ties are like ties through breastfeeding. The same also 
    applies to ties through marriage (in-laws), as mentioned above. The 
    husband’s father is a mahram by virtue of ties through marriage. The 
    husband’s grandfather and the husband’s son are also mahrams by virtue of 
    ties through marriage. Whether (the mahram relationship) is through ties of 
    blood or through breastfeeding, shaking hands is more appropriate.

    End quote. 

    Shaykh ‘Abd 
    al-‘Azeez ibn Baaz (may Allah have mercy on him).