Can my will follow Islamic inheritance principles and still be valid in Ontario?

The formal rules don't change

A will expressing Islamic inheritance shares, or any other faith-based distribution, is executed the same way as any other Ontario will: signed at the end, in front of two witnesses present at the same time, who then sign it themselves, under section 4 of the Succession Law Reform Act. Nothing about the content of a will's distribution scheme changes how it has to be signed and witnessed to be valid.

Freedom of testation, and its one real limit

Ontario gives a testator wide freedom to decide how their own property is divided, whatever the reasoning behind the shares chosen. The one significant limit applies to anyone the deceased was actually supporting: under Part V of the Succession Law Reform Act, a spouse, child or other dependant can apply to court for adequate provision if a will, however carefully reasoned, leaves them without it.

Working through that risk with a lawyer before finalizing shares avoids a court later rewriting an outcome you intended.

A civil nikah or marriage contract and the spousal election

A religious marriage ceremony without a corresponding civil marriage does not, by itself, create a spouse's rights under Ontario's Family Law Act or Succession Law Reform Act; those rights generally depend on a legally recognized marriage. Where a civil marriage does exist, the surviving spouse can still elect equalization under the Family Law Act instead of accepting the will, exactly as in any other marriage, so marriage-contract terms are worth reviewing alongside the will rather than assumed to control on their own.

Executors, guardians and burial instructions

A will can name an executor familiar with the distribution you intend and note funeral or burial preferences, including timing and religious rites, though an executor's legal authority over the body is generally limited to arranging burial rather than overriding a cemetery's or coroner's own requirements. A guardian appointment for minor children under the Children's Law Reform Act works the same way for any family, and still expires ninety days after it takes effect unless the appointee applies to court.

Powers of attorney can carry faith-based instructions too

Under the Substitute Decisions Act, an attorney for personal care must follow a known wish, including one based on religious belief, that applies to a decision about your care, and otherwise act considering your values and beliefs. Recording preferences about medical treatment, end-of-life care, or a faith-based facility in the power of attorney document, not only in the will, is how those wishes actually reach the person making decisions while you're still alive.