Okay, everybody. It's not your problem. I would hate to think I'd be treated or spoken about (like some here's age done) for damaging a pair of $25 scissors. take the high road, okay?
People are more important than things, and the SIL was doing a favor with the deck stuff. I would be disappointed and maybe angry for a short while, but I, too have been guilty of breaking or ruining someone else's belonging by accident or ignorance (meaning not knowing the value of a certain thing). Haven't you? Sure, I'm very protective of my scissors and have trained my family accordingly, but not by humiliating them or casting stones. I've shown them my Ginghers and have scolded them for (long ago) once using the sewing Fiskars.
How how about applying the Golden Rule to this situation which --I repeat-- is not anyone else's problem here. I would let my SIL know in a normal tone ( after I had calmed down or whatever on the inside) why sewing scissors are made only for fabric, and I'm guessing he'd feel sorry and offer to replace them. But I would not insist. I would give grace because everyone needs grace. I'm not gonna bite the hand that's fixing my deck!