What can the church do in a situation where a sister says she has been deceived by a brother who is impotent? He concealed the matter until he had married the sister. Can the sister marry another person since she desires children and was misled?
The short answer is, "no." She married the man. The primary purpose of marriage is to be a lifelong companion (Genesis 2:18). Children are a gift that comes after marriage, but not every married couple is able to have children. Consider that Abraham and Sara did not have children in their marriage until God granted them a child. It took Isaac and Rachel about 20 years before they had children. Jacob and Rachel were not able to have children for many years. Hannah wanted a child badly and she only received one after God answered her prayer. If there were that many without children in the Bible, then it was more common than you might believe. Yet, in none of those cases did God tell anyone to leave their spouse and marry someone else.
If they are unable to have children on their own, and they want children, then look into adoption. There are many children who long for parents and have lost theirs. Turn their sorrow into a blessing for others.