A man may kiss his mother out of mercy, compassion, love, and affection; and this is one of the aspects of righteousness and goodness. Allah (Exalted be He) orders us to be dutiful to our parents, saying:
And your Lord has decreed that you worship none but Him. And that you be dutiful to your parents. If one of them or both of them attain old age in your life, say not to them a word of disrespect, nor shout at them but address them in terms of honour.
And lower unto them the wing of submission and humility through mercy, and say: "My Lord! Bestow on them Your Mercy as they did bring me up when I was young."
A man asked the Prophet (peace be upon him) saying:
"Who among the people is most deserving of my companionship (of kind treatment from me?) He said: Your mother. He again said: Then who (is the next one)? He said: It is your mother (who deserves the best treatment from you). He said: Then who (is the next one)? He (the Prophet) said: It is your mother. He (again) said: Then who? Thereupon he (peace be upon him) said: It is your father."
Kissing sisters should be avoided if the person fears temptation. This should be avoided as a means of Sadd-ul-Dhara'i` (blocking the means leading to sins). May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.