What is the ruling on a mature young man kissing his paternal or maternal aunt, and a mature young woman kissing her paternal or maternal uncle?

Kissing maharim (unmarriageable kin) is permissible under certain conditions: that the kiss is for affection and mercy, not for lust; that it is on the face, forehead, or head, not on the mouth; that there is no fear of temptation (fitna); and that the mahramiyyah (unmarriageability) is based on kinship (nasab). It is recommended not to kiss maharim through fosterage (rada'ah) or affinity (musahara) due to the weaker deterrent. There are texts that indicate the permissibility of this with controls.