Have I committed a sin by preventing my wife from performing Tawaf alone due to my fear for her from the crowding and mixing, and her desire to kiss the Black Stone, and what is its expiation?

A husband's preventing his wife from performing a voluntary circumambulation (tawaf) to touch the Black Stone is permissible and valid if she would be jostling with men. This is based on what Aisha (may Allah be pleased with her) said to a freed slave woman of hers who touched the Stone and jostled with men: "May Allah not reward you," twice or thrice. "Why did you not say 'Allahu Akbar,' make a signal, and pass by? Did you intend to push men away?" Ibn Baz was asked about a woman kissing the Black Stone in the presence of men, and he replied that it is not permissible.