1. Dear *****, If you rent a room out, you will have to give half of any rent received to your brother, as he owns half the house. Additionally, if your brother objects to a room being rented out, you cannot rent the room out. This is because your brother is owner of half the house. As co-owner, he can determine what can be done in relation to the house. So, you need his consent and agreement if you wish to bring third parties into the house. Each co-owner has the right to live in the house as it is jointly owned. However, unless there is agreement between the co-owners, third parties cannot come in and live in the house without the permission of both co-owners.
2. Similarly, if you want your stepson to live in the house, even temporarily, you will need the consent of your brother. This is because your stepson does not have the right to live in the house without the permission of both of the co-owners of the house. Absent consent from both of the co owners your stepson doesn't have the right to live there. So, here, you need your brother's consent to allow third parties to live in his, and your, house.
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