SIL announces her pregnancy at my intimate wedding (a tiny gathering but still important to us, we just wanted one day - I never got to celebrate birthdays or anything like that for myself growing up, whereas she was known to throw parties as she could always afford to)
That was inconsiderate and poor guest behaviour.
When I shared my own pregnancy news years after, 4 months later we found out she had become pregnant despite telling everyone since her first that she did not want more kids, conceiving only a few months after finding out our news.
People are allowed to change their minds about how many kids they want. I highly doubt she got pregnant again to ‘copy you.’
When I had the baby, she came over for 3 days with no invitation as i was in so much pain and not ready to have guests. She brought along her very chaotic toddler, I was still clearly on heavy meds after traumatic birth and still trying to bond with baby. Instead of recovering I was tiding up after SIL daughter while SIL cuddled my baby, took photos as if it were her own child and expected my hubby to entertain her daughter.
Honestly, it sounds like you are a bit of a pushover. There was no need for you to hand your baby over to her for cuddling while you tidied up after her toddler. You need to practise standing up for yourself.
I have discussed this with SO and he is supportive in private but is clearly too scared and set in his ways to challenge her, she has always gotten her own way with parents/ family.
He’s clearly a total pushover too, and that’s a far bigger problem than your ‘nightmare’ SIL.
AIBU or would you consider her a nightmare?
She’s clearly a bulldozer lacking in social and self-awareness, but all of your problems after your first child was born could have been avoided if you had simply stood up for yourself and not allowed her to treat you that way in your own home.