Spendysis take a deep breath.
I'd do nothing at all with the email. If you start sending emotional emails to sister cc'ed to OPG and SS, who it has nothing to do with, you're going to look like the unhinged one. Which is possibly the plan. Discredit you as a witness/complainant.
It looks like your mother's old friends have been recruited into being flying monkeys. If your mother wanted them to be able to contact her by phone she would have put her own phone number in the letter. If she didn't then she didn't and they need to accept it. They can write a letter back to her. Why do they want your or your sister's phone number if it's your mum they want to speak to? And thank her for what? It makes no sense.
Funny how all of a sudden your mother is at death's door just when the OPG start investigating. I'd guess it was your sister who wrote the letter and none of it is true. It'll turn out your mother was in hospital having an ingrown toenail removed or she's been told by OPG doctor that she lacks capacity, or something else that's non-urgent. It's also possible for scammers to use technology that makes it look like an email has come from a particular email address, perhaps your sister has got hold of some such technology. Do your family have your current phone number? I'm wondering if it could be a ploy to get your phone number out of you.
Is mark the email spam so any more goes directly to your junk folder upon receipt, so you don't see it.
Even if it's true, it's for your mother to manage her relationship with her friends, there's no reason for you to be involved. You haven't been keeping in touch with these people there's no reason for you to start now.
It's very telling that the email (and the letter to the friends?) is vague and doesn't say she's dying or what of. Just implies it. It sounds like mind games to me.