Can a will be challenged in Ontario?

Agreed. Where sons and daughters inherit together they take the residue of the estate two shares to the son for every one to the daughter, on the words of Surah an-Nisa 4:11, “the share of the male will be twice that of the female”. Ibn Rushd records this as a point the Muslims agreed on; Ibn Qudama, listing the four kinds of male heir who take alongside their sisters in this ratio — a son, a son’s son however far down, a full brother and a paternal half-brother — states flatly that there is no disagreement about it. We checked this point; the schools do not differ on it. But note what the ratio does not say. It is not a general rule that a woman always takes half of what a man takes: maternal half-brothers and half-sisters share their third equally, male and female alike; a mother and a father each take the same one sixth where there are children; and among the remoter male relatives — a brother’s sons, paternal uncles and their sons — their sisters take nothing at all, so there is no ratio to speak of. The two-to-one rule is one rule inside a scheme, not the scheme.

Shared evidence: Surah an-Nisa 4:11 — “the share of the male will be twice that of the female” · Surah an-Nisa 4:176 — the same ratio between brothers and sisters in kalalah · Sahih al-Bukhari, Book 85 (Fara’id) — “give the fixed shares to those entitled to them; what is left goes to the nearest male”.

Not covered by this finding: whether lifetime gifts may lawfully be used to even out the difference between sons and daughters, and how a Canadian court order varying a will — a BC wills-variation claim, or a dependant support order — sits against the ratio — not yet researched school by school. Treat that as open and ask a scholar of your school.