@Spendysis I'm sorry this is happening to you. I think you really need the extended time off work to get your head around everything. You're not just grieving the physical loss of your Mum, it's also the final loss of hope of reconciliation with her, or even a final conversation.
As you may remember, I think it's very likely I will end up in a similar situation so I have given some thought to how I will handle my Mum's funeral etc. I think I will go, with DH to support me, and probably sit at the back quietly. I know that I will feel angry with my sibling and extended family for similar reasons to you, so I know part of me will probably want to call them out publicly but I don't think I will. Mainly because it will make it easier for GB2 to claim I am 'unhinged' etc. For that reason I don't think I'll attend any gathering after the service, but if any family members seem friendly towards me I MIGHT suggest meeting later. However, if you decide it's too much for you to go I think that's perfectly reasonable too. You could instead do something with your DH and DC in memory of your mum- perhaps visit somewhere she enjoyed, or light a candle and talk about your memories of her- whatever feels right for you.
In your case I suspect part of the reason your DSIS will not have told you about the death etc is in the hope that you won't attend and she can use this as 'proof' that you are the bad person as well as not risking you saying anything that contradicts her story. She probably knows, or will at least suspect, that you know OPG were investigating and she won't want extended family etc to know about that so I doubt she'll try to stir up an argument. Extended family will undoubtedly assume she has told you about your mum's death and funeral arrangements (I can't imagine any normal person wouldn't whatever the history, and they think she is a normal person). I think you'll probably find that a small number of family members are really on side with DSIS and the rest know there is disagreement and don't want to get in the middle of it. Unfortunately for you, not challenging DSIS is the easiest way of avoiding being part of an argument. So it's quite likely that many family members are not as 'onside' with your sister as she thinks, they may just not want to challenge her. My view with my own family is that such people do not deserve my time, but you may have reasons to take a different approach.
I hope it all goes as well for you as possible and can give you some sort of closure on this difficult chapter, whatever you choose to do about the funeral.