She also questions me on so many things (3 decades ago she set her baby out in the sun for 10-15 minutes every day and even though I told her we're not doing that with mine, my FIL caught her sneaking the baby put into the sun when I wasn't looking).

Handle these things in a binary way (Either accept it and move on, or demand that it be stopped or the person in question will be thrown out of the house.)

If you can't do that, it suggests to me that your reaction is less about anyone's well-being than it is about defensiveness/shame, confidence issues, and defaulting to interpreting disagreements as criticism. That's what I'd say is happening here, and that's not so much caused by parenting hormones in particular, as it is a more general ego-sheltering behaviour of people when it comes to things that are important, or assigned critical importance or ethical complexity in society.

So she's taking your kid into the sun. If you trust her not to drop it, and aren't afraid of skin damage or sun allergies, where's the spite really coming from here, other than feeling criticised? Surely, if she can "sneak" out your kid, you quite apparently weren't all that concerned about holding it at that given moment, and I'm sorry to be blunt, but your reaction makes you sound like a toddler who's envious of the neighbour touching the toy it hadn't looked at for a month. The solution is clearly to turn things around to be more accepting about her spending time with the baby, as long as she's up-front about it. And you'll only be able to allow that, if you move beyond feeling criticised for not raising your kid the way she raised hers.

TL;DR: I'd challenge your premise that these phaenomena are primarily biologically tied to parenthood. I think children are just a big deal in society, so it stands to reason that minor disputes about how to do it would escalate. The solution is to keep hierarchies clear, make definitive decisions and consequences, but also stay level-headed enough to know that not every disagreement is a criticism, and even if it was, criticism isn't an attack that should hurt you, and definitely couldn't ever damage you if you don't let it.