Is it permissible to propose marriage to an unmarried male colleague for the sister of one's wife, who is not beautiful, and what are the intentions and regulations involved, taking into consideration the wife's insistence?

There is no objection to offering the wife's sister to a righteous, upright man of good character, out of a desire for good and a pursuit of their chastity. The one who performs this act will be rewarded for it.

This was the practice of the righteous predecessors: a woman offered herself to the Prophet, peace and blessings be upon him; Umar ibn al-Khattab offered his daughter Hafsa to Uthman ibn Affan, may Allah be pleased with them both; and the righteous man offered one of his two daughters to Musa (Moses), peace be upon him. This demonstrates guiding towards good and striving for it, provided the intention is sound and the purpose is pure.