Can someone bring a dependant support claim in Ontario against an estate being administered in another province?

It's possible, but jurisdiction becomes a real, fact-specific question the moment an estate is being administered outside Ontario. A dependant support claim under the Succession Law Reform Act is Ontario litigation, and an Ontario court generally needs some meaningful connection to the estate — assets located in Ontario, an Ontario-based estate trustee, or another jurisdictional link — before it can make an order that actually has teeth against that estate.

If the deceased lived and died in another province and the estate holds no Ontario assets or connections, an Ontario court may simply not be the right forum, and the claim may need to be pursued in the province actually administering the estate instead, under that province's own estate law. Where the picture is mixed — for example, some assets in Ontario and others elsewhere — the jurisdictional analysis gets more complicated and more genuinely uncertain. Because interprovincial estate situations turn heavily on where assets sit and which court can practically enforce an order, anyone in this position should get legal advice early, ideally involving a lawyer in each province involved, rather than assuming Ontario's process automatically applies.