Under normal circumstances, erotic tension is increased when there is an obstacle to acting on it. But now you have an extra challenge, because courting danger in non-virus-related ways is a common behaviour during this time. Like many others, you may be unconsciously drawn to a peril that you can control. Recognise this and set firm boundaries. Do your best never to be alone in a room with your brother-in-law. Stop fantasising about him. Increase your bonds with his wife and children. Work to improve your relationship with your husband and try to reawaken your sex life together. Focus on your children, on establishing routine, exercising, calming your mind and keeping your family safe. All this shall pass.