What is the ruling on a man kissing his wife's mother, or a woman kissing her husband's father, and does that nullify ablution and fasting?

The wife's mother and the husband's father are considered unmarriageable relatives (mahrams), and kissing mahrams is permissible under three conditions. A kiss does not invalidate ablution (wudu') unless something else results from it, nor does it invalidate fasting unless it leads to the emission of semen.