I don't trust MIL alone with DD, but at this point, for a few hours to get things done, I would let MIL watch her. I have asked MIL if she would help us in that way and she always says she'll reach out, but never does. The few times we've asked, she has been unavailable. 

I don't go out of my way to keep DD from seeing MIL, but also don't go out of my way to make opportunities for MIL to see DD. I feel that MIL is dismissive of DD and I don't like it. For example, we had some chocolates in a decorative container on our counter. MIL makes a big show of taking one and popping it in her mouth in front of DD, so then DD wants one. My mother who was also there tells DD, "no, no!" Then DD (20 months at the time) starts to cry. I then turn and say, "she can have one of those" (because we do let her have an occasional chocolate and my philosophy is that unless it's alcohol or something unsafe, my child shouldn't have to watch everyone else eat something and be told she can't have it, just because she's a toddler). So, I give her a chocolate and MIL says, in a snarky tone, "DD always gets what she wants." This is the same woman who always told DH that he had to entertain SSs when they were with him and let them drink as much soda and eat as much junk as they wanted. She also spends at least $400 on each of them every Christmas (and then tells DH she didn't get him anything because she spent so much on SSs) and tells DH he has to "make it up to them", if he ever has to do something other than entertain them on their weekends (or if she or BM schedules over his weekends). So, my main beef with her is that she treats DD differently and as soon as DD is old enough to notice, I'll be done. For Halloween, DH sent a text of pumpkins we'd carved to MIL and BIL. MIL replied, "Are those the pumpkins I gave rhem?" because she thought it was BIL sending the text and apparently, gave pumpkins to his kids. She saw DD in October, but did not feel the need to give her a pumpkin apparently. She will say, "oh, we just didn't expect any more babies", but DD recently turned 2 and she exists, so no reason why she shouldn't be treated like a grandchild at this point. MIL's interactions with DD also seem to revolve around DDs interactions with SSs. She asked me if one of DD's first words was one of SS's names. Um, that SS doesn't interact with DD (it's both funny and sad to watch DD try to catch his attention when he's around) and neither SS spends more than 24 hours a month at our house, so of course, DD doesn't know them. Even yesterday, MIL acted like DD was being rude or dumb because she didn't know SS's name...she doesn't know who SS is because she has seen him for all of 20 minutes in the last month.
 

My MIL is the very definition of toxic. We see her as little as possible. She will periodically complain that she never hears from DH, but whenever he talks to her she either tries to manipulate him, make him feel bad, or wants money. 

I shouldn't have been surprised that MIL plays obvious favorites. It's clear that DH is her scapegoat, BIL is the golden child, and SIL is the one the MIL literally chases for affection (it's embarrassing to watch MIL practically do cartwheels to get SIL to pay attention to her in the few times SIL is around - usually twice a year). And I was the one who came along and told DH that he didn't have to be a stand in husband for MIL...and that I wouldn't put up with MIL treating my husband like he was her husband. Now, she is working on making SS her husband by asking him to come over and do things for her, like hang her Christmas lights and paint.

I should also not be surprised that MiL treats SSs like they are a separate family - making plans directly with BM or with them without even giving DH a heads up - but that is how MIL wields her power...and when DH has called her out on it, she just plays the victim and says he is "always yelling" at her and she "has to intervene" to show SSs "they are loved" and because he is "just at war" with BM. Barf! 

I honestly have no time to give to needy, manipulative women Iike MIL and BM...the more needy and demanding a person is, the more I shut them out. I used to try to be at least a little helpful and responsive to MIL, but she's shown time and again that no good deed goes unpunished, so I'm over it. She has also repeatedly undermined our home and family and I'm over all that. 

Oh yes, she always pays him...and then will sometimes complain to DH that SS did a bad job and expect DH to fix it...as if it's somehow his problem even though he wasn't involved at all. 

I'm sorry you're going through the same thing. It's almost as if our in-laws decided that our DH's got "one family" and they'll just ignore all future children. My MIL always says, "we're just not used to having babies around", but I guarantee if DH's sister had a baby, we'd all be expected to cater to that baby all the time. It's also weird that BIL's kids have little interest in DD. BIL's daughter is 8 years older than DD and will sometimes acknowledge her, but mostly ignores her. I have cousins who are 10 and 12 years younger than me and whenever they were at my grandmother's house, we would beg to go over to spend time with them. I'm closest to my cousin who's 10 years younger...even closer than cousins my own age. At this most recent thanksgiving, SIL and her two kids sat in a room by themselves and talked to SS, but barely interacted with the rest of us, including DD. BIL likes to grab DD and kiss her and she runs away scared because she doesn't know him. No one tries to interact with her on her level or play with her.