What is the kinship between the Prophet, peace and blessings be upon him, and Umm Haram bint Milhan, which justified his entering upon her and placing his head in her lap, and why did the Prophet, peace and blessings be upon him, visit Umm Hani, and is it permissible for men to visit women who are not their محارم (unmarriageable kin)?

Al-Nawawi mentioned that the Prophet, peace and blessings be upon him, used to visit Umm Sulaym because she was his maternal aunt through suckling or lineage, thus she was a mahram to him. Scholars also permitted a group of men to visit a righteous woman and listen to her words, inferring this from the hadith of Umm Ayman and the entry of Abu Bakr and Umar to her. This is conditioned upon the absence of fitna (temptation/strife) and the removal of suspicion.