Sheikh `Abd al-Rahmân al-Mudkhalî
It is permissible for a person to kiss his or her relatives of the opposite sex who are mahram as long as there is no chance of stirring up any sexual feelings by doing so.
Someone asked Ahmad b. Hanbal if it was permissible for a man to kiss his mahram relatives. He replied: “If he is returning from a journey and has no fear on himself from doing so, then he may.”
The proof for this ruling is that Abû Bakr entered his home and found his daughter `A’ishah lying down beset with fever. He kissed her cheek and said: “How are you, my little daughter?” [Sahîh Muslim]
Likewise, it is reported that Khâlid b. al-Walîd kissed his sister.
Scholars advance certain conditions for this permissibility that should be observed:
1. It must an expression of mercy.
2. It must not be on the lips, but instead on the head, the cheek, or the forehead.
3. The reason should be appropriate, like sickness or returning home from a trip.
4. There must be no temptation feared.
And Allah knows best.
Source: Islam Today