What is the ruling on kissing one's father's wife?

It is permissible to kiss one's father's wife (stepmother) because she is a mahram (unmarriageable kin), as long as there is no suspicion (ريبة) or lust (شهوة). The kiss should be on the forehead or the head, not on the mouth. It was narrated from Imam Ahmad that it is permissible to kiss mahrams upon their return from a journey if there is no fear of temptation (fitna), and he supported this by instances of the Companions, may Allah be pleased with them, kissing their mahrams.