Is it permissible to leave a relative—who lives with me in the same house and does not pray in congregation—with my wife and go to the mosque to perform the prayer, knowing that I have no one in the house except my wife and a baby girl who is not yet one year old?

It is forbidden to leave a non-mahram man—even if he is a relative of the husband, such as his brother or uncle—alone with the wife in the house, based on the Prophet's (peace be upon him) saying: "No man is alone with a woman but the third of them is Satan," and his saying: "Beware of entering upon women." A man from the Ansar said: "O Messenger of Allah, what about the Hamw (male in-law)?" He replied: "The Hamw is death." This relative must be informed of the obligation of congregational prayer. If he does not respond, then he must go down to pray, or leave the house until the husband returns.