Buy a spycam and place it in a discrete place. Don't be greedy, don't aim the sleeping room automatically, they could fuck in different places when you are away.
If you wanna catch your SIL naked, buy one of these spycam disguised in a usb charger and place it randomly. You can even buy one of these spycams disguised in an electric razor, and place it permanently in the bathroom.
Enjoy, and come back with your results
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