1. At the outset, the bad news is that a co-owner of property, even of a one sixth share, can live in the property, as in law he is the owner. You cannot get an injunction to remove your brother once the will of your father has been administered and that half of the property transferred into the names of the 3 children including your brother. YOur sister has no rights, as she is the owner of nothing. Currently she may have some notion that she will inherit when your mother dies. However, until your mother dies, your sister has no personal rights. Under the Lasting Power of Attorney she can handle your mother's affairs. This includes attempting to get the property sold. However, the holder of a LPA cannot take any benefit from the affairs of the donor of their Power. This means that your sister cannot move in and live in the house as this would be benefiting her personally at the expense of your mother the donor of her LPA.
2. However, the behaviour of your brother is unlawful to the extent he has changed the locks to the house. Every owner must have a key to the property. So, formally write to your brother and demand a key for both yourself, your other sibling and your mother/sister. In law, your brother has no right to any of the personal possessions in the house. Your father's possessions form part of his will and the executor can enter into the house and take these possessions of your father away for sale or division amongst the relevant beneficiaries under his will.
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Thank you for your response .It is what I suspected but it is good to have it confirmed .