If my sister signed a paper in front of a notary giving my husband and I guardianship of her son, does that actually mean anything?

Its not a court order so cannot be enforced in court. It may help in enrolling him in school, or if you were stopped by the police or other similar matter where his physical presence with you may come into question. If you can spring for an attorney to present a motion for temporary custody order to a judge you would be totally covered. In some counties, like Allegheny, there is "pro se" motions court which assists persons with this sort of thing.