Can a children's aid society investigation be used as evidence in a family court motion for sole decision-making?

It can be, but not automatically or in raw form. A children's aid society's file isn't simply handed over for use in a family court motion; the parent relying on it typically needs the relevant material admitted properly - through a production request, an affidavit describing what happened, or testimony from a worker or other witness - before the court will treat it as evidence supporting a request for sole decision-making responsibility.

Once properly before the court, an investigation's findings can matter a great deal, because the best-interests test asks about each parent's ability to care for and protect the child, and a documented safety concern speaks directly to that. An investigation that is still open, or one that didn't result in any substantiated finding, generally carries much less weight than one that led to an actual protection finding or a completed, documented conclusion - courts are cautious about treating a bare allegation as settled fact. Expect the other side to challenge how the material was obtained and what weight it deserves, so getting the production and evidentiary steps right, with a lawyer's help, matters as much as the underlying concern itself.