Are my brother's clothes and the places he sat considered impure because they were placed in a location where my impure clothes were present?

The default state of your brother's clothes is purity, so it is not ruled that impurity has transferred to them merely based on doubt, for impurity does not transfer merely based on doubt. It is clear that you are afflicted with obsessive whispers (waswasah), and you must disregard them and pay no attention to them, because going along with them leads to great evil.