Is it permissible for a girl to kiss her male cousin (her maternal aunt's son) on his cheek, and if it is forbidden, is it considered a grave sin (fahisha), and what is the reason?

A male cousin (son of the paternal aunt) is a non-mahram to a woman unless he becomes a mahram through breastfeeding or marriage. Therefore, it is impermissible for her to shake his hand, touch him, kiss him, be alone with him, or appear before him without a covering hijab. Her kissing him on his cheek is forbidden because it is a prelude to zina (unlawful sexual intercourse) and leads to indecency. The four Imams have agreed on the prohibition of a young woman touching a non-mahram man, and kissing is even more strictly prohibited. A severe warning has been mentioned regarding a woman touching a non-mahram man. Thus, the girl must fear Allah, not be lenient in this matter, repent, and increase her good deeds.