Although being left out of a will can be emotionally difficult during a time of grieving, as long as the loved one who has passed, had the mental capacity to make a will at the time it was made, they are entitled to bequeath their property to whoever they choose.
There is no legal requirement that a person treat their surviving spouse or children fairly. Challenges to a will would normally require a court action, and success would depend on proving that the person did not have mental capacity to understand the nature of their assets and who they were leaving the property to at the time of the making of the will.
This can be difficult to prove unless there are medical records at that time that support mental incapacity at the time the will was made. If someone left out of the will is a dependent as defined under the Succession Law Reform Act, they may be able to apply for compensation from the estate (e.g. a spouse or dependent child).
If there is a situation where you contributed to a mortgage or any other asset, there are circumstances where some interest in a property might be established on the basis of a “constructive trust” if you contributed to the mortgage, or costs of running the household, or performed substantial care or other services on a regular basis such that it would be equitable to grant an interest in the asset, or compensation. This involves a concept of “unjust enrichment”. A court action would likely be required to establish a right to an interest in the property or compensation for the value of services provided.