In all honesty I think it's likely you're kidding yourself if you think the 11 year old doesn't already know exactly what's happened. I think it highly likely someone outside the family will have told them and even if not they're likely to suss what's happened and why they can't see uncle/go to grandparents house.
So you need to talk to your children ASAP. You may need to explain to the 11 year old that the other 2 are too young to be told the details yet and that while they will be told (whatever it is you decide to at an age appropriate level - personally I think honesty is best as otherwise as they get older they'll just feel you lied to them) they're not to be told anything else yet.
Your parents are NOT innocent, they are fully aware of the facts and they are making a CHOICE to support your brother. Choices have CONSEQUENCES.
I agree collaborate with your other siblings and present a united front to your parents that you will not be visiting their house, will not be exposing your DC to your brother, they will not be trusted to babysit and that this is because of THEIR choices.
I think it's possible at this point your dad may take a stand to some degree too (don't depend on it but he might) because he may be the one more likely to understand where you and the other siblings are coming from.
I've raised my dd in a similar situation. I'm a survivor of csa by my father, with whom I am currently vlc and have over the years at times been nc too. Dd has met him but she's literally never been even in arms reach of him and only ever in my presence (awaits flaming but I can absolutely assure you all he's not been a risk to her as he's extremely physically ill can't walk even, and she didn't meet him until that was the case. She's also always known in an age appropriate way that he is not a good person wit me gradually explaining why. Previously I was completely nc)
My mother didn't & still doesn't believe me. I don't and never have trusted her to assess risk wrt such matters and so she has never babysat dd, never been left alone with her. I've also been very strict on what photos and videos my mother has of dd.
It's caused a lot of tension over the years and of course means dd doesn't have a close relationship with my mum which at times mum lashes out about. I've always been very clear that she only had herself to blame because she refuses to believe me and chose to stay with him.
My siblings my bro has done basically same as me and my sis (while claiming also not to believe me) and has let mum babysit her kids, but never at mums house.
I couldn't prosecute as no evidence. My word against his.
I'm telling you all this because it feels to you like an immediate issue but it's not, it's a long term one. You will never be able to leave dc alone with your mum - that's the next 13 years you're going to have to enforce that. At least. It's a long haul and it's exhausting and she/they will try to wear you down!
They'll promise he's not going to be there, that they'll keep a close eye, that they won't leave the kids alone with him... But because they don't accept there IS a risk you can't trust that they'll stick to any of this. And abuse can and does occur under adult supervision.
Stick to your guns - I partly regret not completely doing so, not because anything untoward has happened but I do feel I gave in to pressure "he hasn't long left at least let him see dd before he dies" well he's still creaking on!
Dd is 18 now and has chosen herself to have nothing to do with him and vlc with my mum - absolutely her choice I have no problem with that whatsoever.
As a sort of aside, and as a bisexual woman PLEASE don't ever equate paedophilia with homosexuality again. It's NOT the same and it's deeply offensive to do so.