MIL is beginning to unravel and baby isn’t here yet

Love this!

how is it not May if the baby will be 7 months in December?

If your baby will only be 7 months old next December you must only be a few months pregnant. This is all too much, too far in the future.

Pacify her with non-committals and 'we'll think about it nearer the time' and move on. A year is a long time.

Yes I know exactly what you mean in terms of there are good qualities too.

If it offers you any reassurance, we found pregnancy and ‘firsts’/big ticket occasions sparked stuff like this. Sometimes a bit of trying to get control & sometimes a bit competitive and overly interested in what my mum does/will be doing for DD’s birthday/how often we see her etc. The rest of the time is quite uneventful though.

DD1 is 4 now and we haven’t had any actual fall outs. I expect as time passes and your MIL feels a bit more secure or the novelty wears off things will calm down a bit, especially if this is her first GC.

I have a Mil who would like have been a complete nightmare if we'd had kids...so lots of empathy. Put yourself & the baby first.

But to me, this sounds more like she's heading for a MH crisis. Which does require proper intervention, if she does spiral out of control.

Lots of robust signposting to MH services would probably be my suggestion to your DH.

I say that as someone who's family ignore this & just are all basically batshit. You can only do what's in your control.

Why aren’t you going to let your MIL babysit once your child arrives?

Your H needs to explain all that. I'm not excusing your MiL, as she is behaving very badly, but paternal GPs do often get relegated into the 'junior' GP role. People hotly deny this, but its absolutely a pattern arising from greater trust by mums for their own mums than their MiLs, and mums typically having the say where the DC are concerned.
Some paternal GM accept this with good grace and understanding. Others don't and there follows a life time of push and pull and (not entirely unreasonable) jealousy.
I think this could be helped with honesty with the paternal GM from the get go rather than it being the elephant in the room with people pretending to be 'equal' and then blaming the paternal GM when she's upset when it clearly isn't.
Clearly you are doing your best to be inclusive of your MiL, but she sounds rather unstable. In which case you may not ever feel comfortable with allowing her her 'turn'. Your H needs a calm talk with her where he sets out her role and takes responsibility himself for that.

My mum was like this. Let your DH deal with her. Dont even try to rationalise with her. She is being irrational and you will make matters worse, believe me.

Did your H explain to his mum why you both asked your mum to babysit and not her? Because if this is going to be the pattern of how you treat the two grandmothers for whatever reason, I think he'd do better making that clear from the start. Get him to explain why she was not your choice whatever that reason was, and she will know where she stands going forward. She will no doubt not like it, but at least it establishes boundaries and manages her expectations.

Your DH needs to tell her that she is jeopardising yours and the baby's health and it must stop or she will not be involved with the baby. It's pointless of her being jealous of your DM, she will have her turn if she behaves.

I was lucky enough to have 4 involved DGPs and probably at times they may have been a little jealous but I loved them equally and my DF really tried to share me equally with them.

Sympathies OP. With your baby due in May (you said they'll be 7mths next Xmas?) to be getting this so early in your pregnancy must be very troubling for the months ahead.

The advice to either mute and let dh respond to the messages, or just telling her to speak to dh and drone on about boring pregnancy stuff are the way ahead. Your dh seems to be doing OK handling it.

If you can sort it at this stage, the 2nd and 3rd trimesters should be much better.

A pp suggestion of long boring rambling replies is bloody brilliant,

also non commital "hmmm I can see it's very hard for you both. He's been wonderful this morning, and have you seen the weather?" Etc

I like this idea actually. She’s making it all about her, and needs to be reminded it isn’t! I’d then just send a ‘you need to speak to your son’ message on repeat, and ignore.

Absolutely this. My first thought upon reading the thread tbh.
OP, you valuing the family is great but when it doesn't reciprocate you just need to stop putting effort either.

Is the lack of friends and family due to her own behaviour?

I would just tell her that the more she messages you trying to stake her claim to your baby, the less likely it is that you will ever feel comfortable leaving your baby with her and, if these messages continue, you will have no other option but to block her as it is affecting your mental and physical health.

Reply honestly, that her messages are stressing you out and that your priority is looking after yourself and baby, so you'd appreciate it if she doesn't drag you into conflict.

Say you look forward to her meeting baby and suggest that if she can keep interactions light hearted, positive and supportive it will be appreciated, and reciprocated, but that if anyone becomes a source of unnecessary stress, you will step away for your / future babies peace and wellbeing.

Who posts a photo !
He or you need to say her bahaviour is making you feel sick and you won't tolerate it when the baby is here.

And don't let people post pics I had to cut everyone off my FB at times when I wanted to keep things diacreet

When you have the baby you must put your self first

How did she find out ?don't let her find out