Are adopted children treated the same as biological children under Ontario's intestacy rules?

Yes. Under Ontario law, a legally adopted child is treated the same as a biological child for all purposes, including inheritance under the intestacy rules. An adopted child has the same right to inherit from adoptive parents — and adoptive parents have the same right to inherit from an adopted child — as if the child were born to them.

Adoption legally severs the relationship between a child and their birth family for inheritance purposes, unless an adoption order provides otherwise (which is uncommon). This means an adopted child generally does not inherit from birth parents under intestacy, and birth parents do not inherit from an adopted child.

Stepchildren (children of a spouse who were not legally adopted by you) are in a different position. They are generally not included as "children" under the intestacy formula unless they were legally adopted. If you want a stepchild to inherit, a will is necessary to name them directly.