Confused

So if u know that your MIL is about to do something to her own daughter which is ur sister in law and ur very close to her and u know this thing will put ur SIL in a rally rough spot would u tell the SIL?? I’m torn between both because I’m close to both and they are at war and I’m caught in the middle of it because I know this is about to happen but my SIL doesn’t........