Your partner has a financial interest in the house to the extent of the investment of the £20 k and the increase in value of the house consequently. The interest is called a beneficial interest and your partner can claim his interest. The issue of how it is paid is between the brother and his wife.
You would follow the beneficial interest in the house to whoever received the proceeds of sale and sue them, if necessary.
I would love to help you but I do not work in a private practice so my rules of employment do not permit me to.
A reasonable sized local solicitor's practice should be able to help. Make sure you get a rough estimate of costs.