MIL made me feel unwelcome.

No one said you do nothing.

we're saying it's weird and selfish of you to expect your hot, pregnant wife to get the toddler home and to bed while you stayed partying with her family.

I voted YWNBU. I breastfed all mine and would not have expected you to have come back with me, pregnant or not. If I wanted you back with me, you'd be coming!
But...if I knew you had had bad blood with my sister in the past, I doubt I'd have left you there alone and I'd have already checked you were welcome, not sprung it on her.
So...I think your in-laws tolerate you for your wife's sake or they like you, but sister and BIL have remaining issues.
Your wife either hasn't been told or hasn't worked it out and, like you, thinks it had all blown over. In your shoes, I'd certainly have taken the hint and done a quick U-turn, yawning, change of mind. Dilemma solved. Tattle with wife on the way back - her family's reactions not being her fault - and finished the holiday peacefully but with guard up for future plans.

This

Tbh Op you don’t come across well here and I can see why. Your wife is pregnant and you happily let her go back to put your toddler to bed whilst you stay out drinking and partying with her family. And when your MIL rightly hinted that’s odd, you caused a bit of a scene at your SIL’s post wedding celebration.

Lack of self awareness definitely comes to mind.

You should have left to support your wife, not stayed while your pregnant wife had to parent solo

This. I find it odd you wanted to stay with the in laws longer.

In general if we do something with my family my husband will look after our child more, if it’s with his family then I look after our child more. So we can each spend a bit more time with our family.

Would your other half ask for a lift before he decided your mother could drive him home or just assume?

If he'd previously fallen out with your sister would he invite himself without asking to stay late on his own?

Would he then start an argument with your family bitching about your mother if he didn't get the answer he wanted?

If so, you really want to rethink things

She was probably horrified that you were letting your pregnant wife go home alone to put your toddler to bed, whilst you carried on drinking and partying with HER family.

She was probably surprised that you were even mentioning staying in the circumstances, and you sound like you were petty and bitchy to top it all off. A real Prince.

OP you're getting flamed here, but I honestly dont see the big deal. When I was pregnant my OH would stay on partying with my family and I was happy to be going to bed, it was never a problem. Maybe MIL was just caught off guard as she expected you to leave with your DW. Try not to focus on it too much, as long as you and DW are in the same boat, that's all that matters.

Have you thought about supporting your wife?

I don't think you need to be lactating to put your child in the bath, then their pjs, bedtime story or whatever. Maybe you could keep your wife company, rub her back, make dinner?

Did you discuss with your wife/SIL that you'd like to stay?

If so dId your wife/SIL mind that you wanted to stay at the party?

I would have very pissed off in your wife’s shoes.

Hoping this isn't real because no one can have so little self awareness right?

Yes this. She said don't know as I think she meant you should be checking with your wife.

Maybe she was judging you for not helping your pregnant wife take your joint child home after a long day.

Ok, in that case they made you feel unwelcome because you are not welcome, they think you're a shitty partner and they are sad she's been dumb enough to have another baby with you.

They don't think you pull your weight with the baby/ are not supporting your wife.

Good on the MIL! Mine always encouraged DH to stay, leaving me to manage the kids solo.

Yeah you still should have gone with her. She’s on holiday and pregnant, she doesn’t want to do everything alone and you didn’t need to stay out drinking. Your MIL just might have had a moment of surprise that you didn’t want to go spend time with your wife and child and would rather stay and drink with SIL who you don’t totally get on with. You were then very rude by calling her out on it.

Right - in which case they were surprised that you were leaving your hot, pregnant wife to go and do bedtime for a toddler on her own while you carried on partying.

I think it was just really weird that your wife would go home with the little one, leaving you to continue celebrating with her family. The normal thing would have been for you to go (unless a drip feed is coming), and they were surprised you didn't.

unless there’s about to be a massive drip feed, etiquette would dictate that you would take baby home (or at least accompany your wife, considering your history with her sister). Even putting this aside you were rude and confrontational in your response
Do you have a tendency to misjudge situations like these, I’m wondering if that’s behind the bad blood with SIL?