Sorry your family was in this position.
An ex-boyfriend of mine was sexually abused by his aunt and his cousin (her son). He confided me in about it when he was older ( we were in our late teenage years). He was very messed up about it, in particular he was worried he would become an abuser himself in later life. I suggested he seek both professional help and family support. And he did.
As well as getting counseling he spoke to his mum. I'll never forget him telling me "Turns out our childhoods have a lot in common". She'd been abused by the aunt (her sister) and their dad, which now that I'm older I probably really see as both sisters being abused by their dad.
It did help him a lot, he and his mum did do some counselling together, it also helped her because she also sought counselling (she hadn't before, she'd never spoken about it to anyone before). They became a lot closer, and well, a lot happier.
He is now married and has a baby. He wasn't on that path in life before he got help and support for what happened to him. I really think he'd have been a drug addict/alcoholic/dead by 30 if he hadn't sought help. His mum credits therapy with saving her marriage, she was really starting to struggle with her buried emotions.
So if you suspect that something may have happened to your sister and that she hasn't sought help, I think there is a duty of care issue here. I do think you would need to talk to your mum first about it though. You will need to approach it with some tact. It may be that you need to approach it from the point of view of your emergent memories, rathe rthan your mum's story, if you don't have your mum's permission to share her story. I do think that the wishes of a victim for secrecy are important here, but I think confiding in and swearing to secrecy someone who is a child of a victim and part of the family situation in which abuse(s) occurred (and where there may have been more, as yet I revealed victims) means those kind of boundaries have already been significantly blurred.
Recently, I told my step-family that the reason I didn't want to see my long-estranged father even though he is currently dying from cancer, was because he abused my mum. And that I witnessed that as a small child.
He had managed to conceal it from them, and to them had always blamed the fact he didn't see me at all after the age of 7 on me/my mum/life being so unfair. When in fact he just stopped showing up on Saturdays. (I tried an adult reconciliation with him years ago. No remorse, said it was my mum's fault he hit her etc, said some dreadful things and told outright lies, so that was a non-starter).
And so, when recently asked to go see him, I said no and I said why. And as soon as I did, all the pieces fell into place. They just didn't know. He told a lot of lies in all directions at that point. Looking back, I think he was terrified that I'd blurt out what he'd done to my mum to his new family and ruin the cosy new set up. It was a very confusing time that suddenly made sense.
It felt very good to say no, say why and realise that in some little way the truth had been set free. I realized that the step-family's hostility over the years was probably as a result of some pack of lies, that me and my mum had been scapegoated so he he could enjoy a new life. It felt good that he wasn't going to get away with it totally scot-free, blaming everyone else for all his troubles anymore. It was probably the single most mentally and emotionally freeing thing I've ever done.