Is this bonkers SIL behaviour or am I overthinking it?

Does this come from her thinking you have different standards to her?
If its not experience, because you have similarly aged kids..
I'd probably rescind the offer unless you're certain it's just her being a bit anxious

It sounds like she has post natal anxiety. Maybe speak to your brother and see how things are. Presumably she's seen you with your 3 and trusts you enough to say yes. If it's just her explaining baby has a cloth and water not wipes or prefers bottle at this temp i'd humour her as she's clearly struggling.

This is so strange. I wouldn’t be encouraging this behaviour. It’s completely rude. I’d say no that’s far too awkward and maybe you’re not ready to leave baby. Offer is there for when you’re less anxious.

I wouldn’t take it personally, it’s just new mum anxiety, it’s primal.

Yep a bit bonkers!

Tbh I 95% breastfed so I'd want to check how to give a bottle as I would be rusty on paced feeding etc. The nappy thing is odd unless you have only boys and she has a girl and she thinks you'll wipe the wrong way (even then surely common sense)!

Probably just being PFB but if you like her and want to help her just go along with it.

Maybe ask if she can demonstrate to you as opposed to being observed?

I just would withdraw the offer. This is just the beginning. When she goes out, she will be calling you every five minutes, will probably leave you a manual of do’s and dont’s, and will want a run down of what you’ve done on her return. Clearly isn’t ready to leave her pfb with anyone yet. As they say, no good deed goes unpunished, and it’s all looking a bit rude and drama filled.

I think if I liked her & wanted to help her & my brother go out I'd do it this once.

I wonder if you'll "fail" though Op so that she can't go?

What does your brother think?

What happens if you fail the nappy test?
Will she give you feedback and another chance?

Fingers crossed you don't fail!!😅

"She accepted on the condition that she first observe me changing baby's nappy and giving her a bottle to just make sure I do it it the same as her.

For context I have 3 children one of whom is still in nappies."

She sounds nuts.

I would withdrawn the offer based on her lunatic-type conditions. She can stick them where the sun dont shine!

Oh dear. It does seem a bit much but I'd try to think of it as just a way to reassure her that baby is relaxed with you doing these things and id do it once. If she was critical or rude or anything along those lines about your technique I'd probably withdraw my offer but I'd do (almost) anything I could to help this go smoothly.

She’s probably just anxious and if you humour her, the whole build up and worries will be far easier for her.

I'd rescind the offer. Her baby's 8 months old not 8 weeks, she'll have to unclench sooner or later. Why does she care about bottle feeding methods at this age? Does she need to see a GP?

Definitely just anxiety about leaving her baby for the first time! I used to get so stressed about how my DH did our first borns nappy slightly differently to me, utterly ridiculous in hindsight! Go along with it and she will level out eventually!

I'm shocked at this! What the fuck!

I wouldn't do it because it sounds like she isn't ready to leave her baby and I would be very worried in case anything at all did happen when the baby was in your care.

Or you know you put the nappy on upside down or something ;)

You must remember how things felt with your first baby.

she’s just an over cautious first time mum.

I think my reply would be 'If you feel you need to check the nappy skills of someone on their 3rd child then honestly, you're not ready to allow anyone to babysit'.

Think that's kind of where I would be on an answer, especially if my youngest was still a babythemself!

Difficult one to vote on, but I voted YABU unreasonable simply for not taking into account she’s a first time mother, she is likely a squirming ball of anxiety and regardless of you having 3 kids, she’s having a hard time coping with the thought of leaving her only baby with someone who is not their mother.
Yes she’s “technically” being irrational, but that’s ok. She’s new to this and she’s panicking. I’d suck it up, change the nappy in front of her and try to put her mind at ease.
I say this because I was the same when my DS was born. I was petrified even to leave him with my mum, and I was too scared to let anyone else push the oram in case they let go or tripped….. I have terrible anxiety and I’m always catastrophising stuff in my head-being a new mum increased it 1000%. I knew I was being insane, but I’d have been incredibly hurt if anyone had actually said as much to me

I'm wondering what she'll do if she decides your skills are lacking? Teach you the 'right' way to do it or just not go out?!

Maybe narrate yourself doing it in your best David Attenborough voice..
And here we have the human infant lying quietly awaiting it's clean nappy....
Now we see the human infant glugging happily on the perfect temperature baby bottle....

I don't even have kids and my friends with babies would leave me with them without a second thought, let alone an observed assessment.

I think you should fake up a certificate for her to award you afterward.

Very strange. But she’s probably just very anxious.

It’s definitely a bit extreme but harmless and I’d not take offence. The bottle thing makes more sense as people do forget and all babies are a bit different and she may have picked up techniques that work for her to reduce air-intake, post feed reflux etc. You were an expert on your babies, she is an expert on hers.

Bless her!

Also remembering the time at 6 months old they put him in a forward facing car seat with only a seat belt. Badgered me to feed him solids at 6 weeks.

I’m sure loads of people in their family thought I was PFB.