Broken inheritance promises generally fall under the law of the doctrine of proprietary estoppel- the leading case Cowper-Smithv Morgan 2017 SCC 61 sets out the general principles of which are as follows:
An equity arises when:
1) A representation or assurance is made to the claimant, on the basis of which the claimant expects that he will enjoy some right or benefit over property;
2) The claimant relies on that expectation of inheritance by doing or refraining from doing something, and his or her reliance is reasonable in all the circumstances;
3) The claimant suffers a detriment as a result of his reasonable reliance, such that it would be unfair or unjust for the party responsible for the representation or assurance to go back on his or her word.
The representation or assurance may be express or implied. An equity that is under crystallized arises at the time of detrimental reliance on a representation or assurance. When the party responsible for the representation or assurance possesses an interest in the property sufficient to fulfill the claimant’s expectation, proprietary estoppel may give effect to the equity by making the representation or assurance binding.
Where a claimant has established proprietary estoppel, the court has considerable discretion in crafting a remedy that suits the circumstances. The claimant who establishes the need for proprietary estoppel is entitled only to the minimum relief necessary to satisfy the equity in his favor, and cannot obtain more than he expected.
There must be a proportionality between the expectation and the detriment. Estoppel claims concern promises which, since they are unsupported by consideration, are initially revocable. What later makes them binding, and therefore a revocable, is the promisee’s detrimental reliance on them. Once that occurs there is simply no question of the promisor changing his or her mind.
It is the promisee’s detrimental reliance on the promise which makes it irrevocable. The detriment need not consist of expenditure of money or other quantifiable financial detriment, so long as it is something substantial.
Generally speaking, the approaches to determine whether a purported repudiation of the promise or assurance is unconscionable and all the circumstances. The issue of detriment must be judged at the moment when the person who is given the assurance six to go back on it.