@Flowerfairy101 I think you probably need boundaries with consequences. She's not respecting your boundaries because she doesn't suffer enough when she repeatedly tramples over them.
You tell her you're not discussing your sister. Leave the room if she continues.
If she brings up Penelope Leech, say I'm not discussing this, I already told you, and leave the room.
If she criticises your parenting, tell her she is not welcome to give unasked opinions. Leave the room if she continues.
If she shouts at you, insist that this is completely unacceptable and you will not tolerate it. Seriously, think about it. How dare she! I let my "mother" scream at me for years because I didn't realise I deserved better. I mean, who the fuck do they think they are?!
If leaving the room isn't enough, then you need to threaten further consequences - "if you persist with this behaviour you will need to leave my house and go home" - and you must follow through.
You are giving her the opportunity NOT to behave abusively. If she doesn't take the opportunity, that's on her, not on you.
Everything about your relationship is about her. What about you? Don't you matter?
You've been trained by this selfish bitch to have your own needs walked over by her. Reasoning doesn't work with her, she doesn't care. She actually doesn't believe that you have any rights. You are not your own person. You are the person she insists you be. But you have the right to be your own person, you have the right to have your needs and feelings respected.
I sympathise, I tried and tried with my "mother", she just got worse and worse, I reduced the time I spent with her and tried to support from a distance, ordering things online for her, keeping track of her banking, etc. but eventually she was so vicious to me, she made up a row and started shouting things designed to really hurt me - simply because I'd been away for the weekend and told her I was turning my phone off (she never phoned unless she wanted something but it seems I was to be available at all times). Having criticised my parenting, and my children, for over 20 years, she then told me if I'd been a proper mother she'd have a better relationship with her grandchildren. That did it for me. If you wait long enough, your mother might say something that makes you finally walk away. But I wish I had walked away many years before. Instead I let her abuse me for decades more than I might have experienced otherwise.