My mom has no boundaries. And she does that whole passive-aggressive talk about you to your sister behind your back thing with me and my sister too. I make a habit of never telling my sister what she says and if my sister tells me anything my mom said about me to her I just pretend it never happened. My mom isn't perfect and I don't want to raise my kid the way she raised me so I don't really care what she thinks about my parenting. I pick my battles and when I started I started small. She backs off more but for a long time dealing with her was the worst. No one is ever going to think everything you do is perfect and some people are wired for gossip and drama. I like to lay low and I try hard not to judge anyone because it's not good to pay that much attention to what everyone else is doing "wrong." Whenever my mom is overstepping a boundary and I can't decide how to handle it, I try to remember that her childhood was way worse than mine and that she did the best job she could raising me. I didn't have a perfect childhood but I know she loves me even if she doesn't always show it in the ways I would be more comfortable with.
You will never be able to change her or her behavior towards you and if you're looking for validation as a mother from her, that might not happen either. But you can change you. You can change whether or not you allow her opinion of you to bother you, or define how you see yourself. You can decide if her opinion of you and your actions is more valid than your own opinion. Unless she is by your side all day and night all the time I really doubt she has any room to judge what kind of mother you are. If none of that helps, when I feel really low about my mom and the way she is attacking my parenting, I sometimes say to her, if you wanted me to be a better parent you should of done a better job raising me. But I am in no way suggesting you manipulate others like they do to you. Seriously. That would be wrong.