@costcocosmos
Your MIL is a mother, who has more than one dc. She knows more about sibling rivalry and the possible issues when a newborn is brought home than you do.
I think she could be useful, and helpful. Your older dc is much less likely to be jealous if grandma is there to make a fuss of him. Chances are you'll be in pain, and not up to much and will appreciate the extra help. It's not worth getting het up about.
I almost always agree that as the pregnant woman it’s your choice, but very gently, I agree with this; YABU.
You haven’t had two children yet, and I could be totally wrong and this is just how you phrased things, it sounds like you have definite plans about what you’re going to do (fix DS’ jealousy issues in a day by showering him with attention) when the fact is that the baby may be demanding a LOT of your attention and you may not have any choice about giving the baby more attention than you give DS; in fact, you may be exhausted from the c-section or unwell, you don’t know baby’s temperament yet, and you may not be able to give DS almost any attention at all that day. Having your MIL there to do what you want (if that’s fuss over DS) could be helpful. If she wasn’t this involved in your lives (apparently happily), I might say differently. AND if you weren’t already making plans you might not be able to do alone (and I mean, just you and partner by “alone”). But it’s a very different MIL relationship because of that.
Also, I’m not saying this to be cruel, but you still seem to be very “precious first baby” about your son when he’s already five and you’re about to have two. I appreciate that at least you admit you’re an overbearing mum; I can’t remember any of my friends mentioning part of the hospital experience for their second being sadness that they had to spend just a single day away from their first, especially after 5 years, only if maybe their first was still too little to understand (I know one friend was sad, understandably, because she ended up not being able to see hers for a week due to complications) and I also don’t remember anyone planning “a very special day” for their first, beyond just trusting the caretaker would have fun and love on them. Of course I think you’ll love your children equally, please DONT think I’m saying that at all, but you may have spurred this jealousy on by treating DS too much like a PFB. Perhaps the last thing he needs now is a ton of extra fuss, or you’re going to have a nightmare on your hands when you can’t keep up the same level of attention due to a newborn needing you, when it’s just you, DS, and baby (maybe you get a totally laidback baby; maybe you get a Velcro baby with colic - it’s part luck).
I don’t mean to upset you, and if I’ve misunderstood you, sorry about that. I wish you the best of luck. But yes, it sounds like your MIL will make your plans work out, not hinder them.