Transferring money within our family

My brother is about to buy my mothers house from her for approx. 100K. My mum will then move to live in a Granny flat on my sisters property, she will not pay any money for the use of this flat.
My brother currently lives with her in the house he is about to buy and will remain there once she has left.
Is there any tax to be paid on this transaction?
In case its relevant, my brother is a farmer and will be borrowing the money to fund this. My mum is a retired widow on a state pension, with no other income and has limited savings .

Also just to add... the house is currently in my mothers name and the deeds will be transferred to my brother as part of the transaction process.
This will be my brothers first house purchase

I think that it is as follows
The property will have to be valued. Your brother is entitled to about 500K from your mother so he will only have to pay tax if it is valued over 600K. This value will be the value held to be the acquisition price if your brother ever goes to sell the house so it should be accurate.
This value will also be the value on which stamp duty is calculated but I think that he will only have to pay 50% of the normal stamp.