you say the tounge and grooved slats run "perpendicular" to the house. Strange, but in 1916 who knows. But then later you stated that the roof joists protrude out under the eaves. I would then guess, that the tounge and groove slats are not nailed to the joists. The joists are running perpendicular to the house so normally the slats would be parallel to the house. apparently the slats are nailed to another sub ceiling if you will. Your stud finder therefore might not be reading deep enough our just cannot go that far. If you went out and bought that new electronic stud finder named Zicor you could set it on deep scan and find the studs. They sell for about $60.
What I would do is: Get a drill with the smallest bit you can find but about 6 inches long. You know the general area of the studs from the protrusion out of the eaves. Then start drilling around in that area until you hit one. mark that spot and then measure out or away either 16 inches or 24 inches. But, then the protrusion of the joists will also tell you the spacing.
I think what your are encountering is a greater distance from the tounge and groove ceiling to the actual ceiling joist. You could be two inches from the stud.
When you find this stud for the anchoring of the swing. The swing will weigh about 40 pounds, two humans total 350 pounds a gross of maybe 400 pounds. Make sure that the span of the ceiling joist is not too great from the house to the outer supports. You do not want to stress that one joist. Since you do not know if its a 2x4, 2x6, 2x8, or what.