Oh Nicoles, you are seeing your mother as other than your mother and it appals you.

First, give both of you space and meet, as suggested, in a neutral space. If when setting up the meeting she says she's bringing 'friends', politely say oh, I forgot and have other plans for that day. Eventually, she'll get the idea that you want her alone and not with her friends. Never, ever say it outright to her as you really have no place in judging your mother. To do that simply sets up for your mom a reason to be angry at you down the line.

Secondly, your sister's relationship with your mom isn't your business, it's your sister's. If she can't speak up for herself, which is what you should foster, then help her learn the words she wants to say.

Third, walk away when your mom starts speaking ill of your dad. Don't get into an argument, just find you need to be elsewhere. Again, your mom will get the hint and stop doing that without, again, having an axe to grind with you and your siblings.

As to the whining that will come from your mom that you guys are never there --- offer to be there whenever she asks. Mother's Day, her Birthday, and the Holidays are a must - no matter who she has in attendance. Other than that, you're busy if she includes folks you aren't comfortable with.

Eventually, like every mom, she'll want her babies around much more than the crowd she has now. She's simply enjoying her new freedom and that will look nothing like the mom you've always known. Accept that and save yourself a heck of a lot of grief.