The kids don't know that their father is raping their mother several times a week, and has abused and groomed her until she doesn't even know which way is up anymore, and thinks initiating her own rapes would be 'better'.
If you were in your mum's position, and your daughter was in your position, would you hope she didn't tell you about her pain and fear, and her need to be mothered and supported because it would devastate you, and you should be enjoying your retirement? Or would it break your heart to think she didn't tell you, and struggled on alone?
What would you want your daughter to do, in this situation? Would you want her to stay quiet, to protect her husband from the consequences of his abuse, and stay with him, allowing her to continue to be raped, and her children to be raised by a rapist?
Or would you tell her that it doesn't matter what she thinks she feels for him, how much she feels she loves him, she doesn't deserve to be raped and abused? Would you tell her she can't live without him? Or would you tell her that in a couple of years, she'll be strong and happy, and wondering how she ever survived living with him for so long?
From what you've said about your mum in this thread, OP, I think perhaps you should try to tell her, and let her do what mothers do best - take care of you when you feel like you can't do it yourself.
You said you're afraid to tell her because then you won't be able to unsay it - but that's what you need. To not be able to hide your head in the sand anymore. It's terrifying, but it's bloody liberating once you come out the other side.