She might find out about the tape. Or she might not. It is not guaranteed, but she will know for sure if you tell her.The problem is that Jade threated him. Stephen threatened him. He pulled Derek into his lies. And he knows she WILL eventually find out because the video of him fucking Jade already got leaked.
You'd be effectively kicking her down while she's at her lowest, as opposed to theoretical possibility of Sage finding out at some point down the road, when she's feeling better. I really don't think telling her now is in any way more beneficial for her.
If either Jade or Stephen want for some reason to inform Sage that her adoptive mother fucks her students, well, that's on them.