I've been lurking for a few months. How I wish I had found this community years ago when I had my first LO. I will try to keep BG short and then post MOO's response below in comments because...long. (What is it with BSC and text novels?!)
My mother is a narcissist, so our relationship has always been troubled. By the time I was 11, I was wishing my parents would divorce so I could just live with my dad forever. Well, when I was in my early 20s, they finally did divorce. I CO her shortly after the divorce. The CO lasted about 2 years until I moved back to the area to be around other family because I was pregnant with my first and also wanted to help my sister escape my MOO's home as a single FTM. I reestablished contact without addressing the past (I am a truly impressive rugsweeper, trying to reform), got my Sis and DNe out of there (whole other story), but kept contact because MOO had proven to be trainable. She tried to manipulate me once and got shut down real fast and knowing I have no problem with CO, she tries to be on her best behavior around me now. I know she has not truly changed because of how she treats my sister still.
FF 3 years later to one week postpartum with my second LO. My MOO has never been alone with my children, never will be. I managed to avoid that conversation until this point. She was at my house to visit and 20 minutes in 
, she realizes my DD is not there. She asked where she was; I told her SIL had taken her to play with her cousins to give us alone time with one week old DS. She asked if she could do that sometime. I said, "We'll see," trying to deflect as usual. She can't resist picking a fight when all the attention isn't on her (she did the same to my Sis after her birth in the hospital), so of course she picks now as the time to directly ask, "Why not? Is there a problem?" I take a deep breath, and JADE (pre-DWIL days) with two very concrete examples of her disrespecting my parenting and undermining me that happened to involve candy and TV when my DD had already been told no. I can give the specifics for llama's sakes if necessary. This was a big step for me, and only the third time I had to directly correct her since reestablishing contact. I burst into tears right after she stormed out. She then texted me (posted below in comments. My phone is messed up so I will type as fast as possible) a bunch of BSC that never addressed what I actually said. Then tantrumed my Sis (poor proxy, I felt bad) about it all weekend until my Sis finally told her she wouldn't discuss it any longer.
I'm not proud, but I was one week PP...I rugswept and never brought it up again. This was almost a year ago now. First question: Do I bother directly addressing it now? The good news is she is more respectful of my parenting, asks permission before she feeds DD anything, and hasn't brought up trying to babysit again. The bad news is the holidays are coming up, and I need help with direct non-JADEy answers to give her when she tries to pressure us about celebrations "on the day of" because that is important apparently 
Up until now I have managed to do the same as the babysitting- brush off, don't answer directly, JADE, etc. I need to be prepared with direct answers ahead of time so I don't get caught off guard like I did with the babysitting. Second question: Help with direct responses to guilt trips/pressure? You guys are great at it, so thank you in advance!