I don't want to drip feed, but I also don't want to give too much information out as this is quite an unusual situation.
Please be kind as I'm a bit of a hormonal mess and this is really long.
I have a MIL who likes to be involved as much as possible with our dc, however, it can sometimes go awry.
She has ended up being heavily involved with her dp's children and their partners/grandkids etc. Often to the detriment of her relationship with her son (my DH) and her gc (our dc + other gc in other parts of the country).
One of the girlfriends of her DP's son is very poorly a lot of the time and has some emotional issues, quirky behaviour, and somewhat crazy behaviour.
I was initially friendly with the lady at first but a lot of her "stuff" (for want of a better word) started to negatively impact on my life and, most importantly, my child's life. SO... as you can imagine I've tried to distance myself as much as possible but as politely as possible.
I blocked her on social media but she keeps creating new accounts to find me, and randomly speak to me much to my annoyance 
. She also managed to get hold of my new mobile number a few weeks ago and call me and grill me about potty training of all things!? I
When I had my dc1 I was initially offered a lot of support from MIL, but then this other lady ended up getting pregnant unexpectedly at a similar time and any support I had got taken away pretty quickly.
It ended becoming a little bit.... tug of warish, with my MIL during pregnancy and with the babies (this lady calls MIL "her mummy" etc which I find so bizarre, she would also see on MIL's iphone tracking she was at our house and phone her every time she came to see DGC, when MIL she switched this off she would ring her mobile constantly until she answered).
It culminated in her calling in "crisis" numerous times she was meant to spend with my dc, helping us out, babysitting, general day to day family stuff. We had to come home early from our anniversary dinner (first time out in 2 years) because she rang up MIL in hysterics which ended up being an argument with her boyfriend over a hi-fi speaker.
In the end, I gave up trying to actively have a relationship with my MIL because of it. She would moan (MIL) but wouldn't do anything to stop the other lady's behaviour.
DH and I drew a line under it and decided if that was going to be par for the course so be it. MIL got narky with me and assured me that her gc was more important but it was a case of her winning our trust back having been let down numerous amounts of times. She has started over the last few months be reliable and we've, in turn, had more to do with her again.
When we found out that we were expecting dc2, we were totally over the moon as you can imagine. The other lady then found out from MIL's partner and apparently said "Oh how wonderful if we could have kids similar age again!"
She had been told she could not have any more children, her boyfriend (DP's son) had said he didn't want to have any more children, she was also told by medical professional having children could kill her.
She then confessed to messing with her contraception and lo and behold she became pregnant, against everybody's wishes and advice. She apparently got quite angry that nobody was pleased for her and then proceeded to organise herself a huge baby shower (this was at about 8 weeks pregnant!)
The whole of her pregnancy she has been having liver issues, heart issues, horrendous sickness, she's signed off, passing off her child to be watched by other people (her child also has learning difficulties, aggressive behaviour and is currently being assessed, he's not allowed to go near my dc because when they have met dc ended up with a black eye) .
She has been told if she MUST have the baby at 28 weeks via c-section, otherwise she will have multiple organ failure and die.
She has to go in weekly to be seen and is asked each time if she wants to end the pregnancy but she has stated that they cannot "make her" do anything. (Not up for debate what's right or wrong here, but just to highlight the severity of the situation overall).
Her C-section has now been booked and it's own my due dat. I've asked my FIL who is the only person we can really ask (or trust) to watch DC1 (my parents are deceased, other family live far away, no real friend network that suits the situation).
I will not, however, ask MIL for help/childcare for labour etc.
MIL has got really angry with me because of this and has told me she will not help this other lady whatsoever (childcare, support, general day to day stuff) because she is a "stupid girl" and has "made her bed", DP agrees etc and because of that I should let her take DC whilst I'm in labour.
And whilst I agree that the other lady is probably a couple of sandwiches short of a picnic, I think my MIL is wildly underestimating how much support they really are going to need to give her, the baby and the remainder of the family in the long run. Whether or not she agrees with the situation, but in general (and being a good human being!)
MIL is now getting narky with me (just what you need in the third trimester ) because I won't ask her to have dc1 when I go into labour and I am apprehensive to make any other arrangements for her to do with childcare (she takes DC1 occasionally because she wants to spend time with them rather than us needing childcare).
I don't want to commit to using MIL when a) she has let down my dc1 numerous times b) has let me down numerous times c) is probably saying what she thinks is what I want to hear and d) is wildly generalising what is essentially a very, very sad situation with a very poorly baby, no matter how things have come to this point.
DH doesn't want much to do with MIL, he's very much "you cross me, you're dead to me" and they've never had a brilliant relationship.
Should I stand firm with our arrangements knowing that, ultimately, she is being daft they will end up helping out this lady and his son, and their family, loads or allow her the "benefit of the doubt" and arrange some backups.
Without meaning to sound selfish af, especially considering the situation, it's adding a whole other level of confusion and stress to what should be a happy time, and despite doing my best to distance myself from this individual their "head in the clouds, denial" behaviour is once again impacting negatively on my child(ren). And I feel awful for being angry at her, but I am.
OK, ready to get flamed now! hides