What is the way to atone for the sin of being alone with, caressing, and kissing the paternal uncle's wife, given the fear of exposure if he is informed, and is concealment and repentance sufficient, or is it obligatory to confront the paternal uncle and seek his forgiveness?

The expiation for this sin is sincere repentance: by abstaining from it, regretting it, resolving not to return to it, and increasing good deeds. You both must conceal what happened and not inform anyone about it, because the Prophet, peace and blessings be upon him, said: "All of my ummah are forgiven except for those who publicize their sins," and he also said: "Whoever commits any of these abominations should conceal himself with Allah's concealment." Moreover, publicizing it engenders discord and resentment. It is also recommended to frequently supplicate for the maternal uncle and to be good to him.