en_tn_condensed/gen/38/08.md

Go to your brother's wife

This is a polite way of telling him to have sexual relations with his brother's wife

Do the duty of a brother-in-law to her

This refers to a custom that when the oldest brother dies before he and his wife have a son, the next oldest brother would marry and have sexual relations with the widow. When the widow gave birth to the first son, that son was considered the son of the oldest brother and he would receive the oldest brother's inheritance.