I made a valid will setting out how I want my estate distributed upon my death. Can someone claim more?
Perhaps. If you have dependents to whom you provided support immediately before you die, or to whom you were legally required to provide support, a Court may order your Estate to pay to them more money if the Court is satisfied that you did not provide adequately for the person in the Will. This may result in other wishes in your Will not being carried out as money may have to go to the dependant, or assets sold to raise money to pay the dependant, in priority to those other wishes.
Dependents can include: spouses, same-sex partners, parents, children, or, siblings to whom you either provided support, or were legally required to do so, prior to your death. A spouse includes someone who was married to you and someone who, although not married to you, lived in a relationship of some permanence for at least 3 years (example: common-law spouse), or were in a relationship of some permanence and are parents of a child from that relationship.
When considering whether adequate support was provided, the Court will consider a broad range of factors such as: the dependent's needs and their ability to contribute to those needs (now and in the future); the size of the Estate; the length of the relationship between the parties; how close the relationship was; whether there was a breakdown in the relationship; and, any signed written statement that you made explaining why you decided to not provide more support to a dependent. The Court may also consider whether there is a “moral” obligation based on society's expectations for you to have provided additional support in the circumstances to a dependant.
In the end, although you may have a proper Will that is enforceable, if you did not make adequate provisions for those to whom you owed support, they may get it anyway.