In BC, step children have no right to inherit https://onyxlaw.ca/inheritance-law-bc/ from their stepparent’s estate, unless the step children are specifically named as beneficiaries in their stepparent’s will. So, if the surviving spouse changes their will after the death of the first spouse and doesn’t make provision for the children of the first spouse, the children of the first spouse are out of luck.
Step children have no right to challenge a step parent's Will. Only spouses and natural children or legally adopted children have the right to apply to vary a Will to get a share of the estate.