What is the ruling on greeting mahrams with kissing and shaking hands, and who are the mahram relatives, and does this include mahrams by suckling?

There is no harm in a man greeting his mahram (definition) relatives and a woman greeting her mahram relatives with a handshake or a kiss. Mahram relatives are those mentioned in Surat An-Nur (24:31), and they include fathers, grandfathers, sons, brothers, maternal uncles, and paternal uncles. It is permissible for a man to kiss his mahram relatives (such as his paternal aunt or sister) on the head, nose, or cheek. Kissing on the mouth is disliked, except for one's spouse. This applies to mahram relatives by blood, by suckling, and by marriage, based on the Prophet's (peace be upon him) saying: "What is forbidden by lineage is forbidden by suckling."