Can a court-appointed guardian override the wishes of the incapable person's family in Ontario?

Yes, within the scope of their court-appointed authority, a guardian's decisions generally take priority over the preferences of other family members who aren't the guardian, since the guardian is the person legally authorized to make the relevant property or personal care decisions for the incapable person. A guardian's job is to act in the incapable person's best interests, and to follow that person's own known wishes and values to the extent they can be determined, rather than to satisfy whichever family member disagrees most loudly with a particular decision.

This authority isn't unlimited or unaccountable, though. A guardian who consistently acts contrary to the incapable person's own previously expressed wishes without good reason, or who otherwise appears not to be acting in that person's genuine best interests, can face a challenge from concerned family members, potentially including an application to vary the guardianship's terms or to remove the guardian. Ongoing, serious family conflict over a guardian's decisions is itself something a court can be asked to address. If you disagree with how a guardian is exercising their authority, get legal advice on whether your concerns rise to the level that would justify formal court intervention.