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Please note that I have assumed that the Grandson and the Grand parent hold the properties in equal shares
1. In that case the grandson would own 50 % and the other three would own 16.66% each. This would be registered at the Land Registry.
They would all technically be entitled to live in the property and any one of them is able to force a sale
2. If there has been no legal adoption by the step father then the two nieces will share the inheritance
Please ask if you need further details
A sale can be delayed because unless all agree OR there is a Court Order then no sale can happen.
If there is some other evidence that reference was made to a post adoption then that could be successful
Any owner can apply for an order for sale and it will be granted.
You will need to have evidence that at some point there was a clear reference to an official adoption
If one of them wants a sale the court will order it - no matter who wants what
1. Under the intestacy rules he would not inherit and accordingly an Inheritance Act claim is the only option
2. He will need to use the Act - the other option might be Proprietary Estoppel if she had made promises to him about an inheritance
It would be more difficult unless there was evidence of a previous Will