Is this bonkers SIL behaviour or am I overthinking it?

It’s a lot OP but it’s her first baby and she’s probably just anxious AF about going. It might just help ease her mind a bit! I’m sure you’ll laugh about it in years to come.

The bottle thing I wouldn't mind so much - I can think of various reasons - but the nappy thing is nuts! I think you are right. Grit your teeth. It is most likely her anxiety at leaving the baby talking. However, I'd be mental noting it in case it is a sign of things to come.

This entirely. In your SIL's defence, I am aware of some OTT (IMO) behaviours of new parents.
I would feel really peeved by your SIL's suggestion that she observes you doing something that you have done 10000s of times before but I suggest you paint a smile on your face while she observes you

I think this is very gracious of you. As PP have said it is definitely more a reflection of her anxiety rather than of her thinking you will be a sub-par sitter. I would certainly be a bit irked by it especially as it has been framed as she’s checking you’re doing things right rather than there’s a few things she would like you to do- but agree don’t make a big deal out of it and humour her.

You’re doing her a big favour as the more she gets used to other people having her child hopefully the more relaxed and chilled out she will be about it. If she’s planning on child going to a childminder / nursery at some point she will have to get used to the idea of people doing things differently to her and that her child will survive or even thrive in this setting.

Perhaps could be a good time for you to check in re her mental health / anxiety / how she's coping with it all. And hopefully you can definitely laugh about it in the future

@Orangelight23 I see you’ve said you're going along with it but I hope you’re comfortable enough to say something like “I’ve done this for 3 kids of my own so hopefully I’ve not been doing it wrong all this time. And will you be testing my DH since nappy-changing isn’t just women’s work?” In a fun jokey tone of course …

Is she otherwise quite a reasonable person? I agree it sounds weird to put conditions on family babysitting like it's an interview. She could have just said do you mind coming over one day for a few hours for lunch so I can see how baby is and put my mind at rest before leaving her. Unless the baby is prone to something like acid reflux or nappy rash and there's a reason she may be so particular?

Honestly I sound like your SIL. My PN mental health is in shreds and I can’t bring myself to leave my 8mo with anyone! She clearly wants to be ok with it. Sounds like checking (intrusive thought coping mechanism) so it’s nothing personal to you. People might be saying it’s OTT but it really sounds like she’s having a hard time ❤️

Also - so many unnecessarily harsh responses on here! Calling her a lunatic, batshit, saying you should withdraw all offers of help. Mumsnet can really be a pit of vipers sometimes. So much for women supporting women! She's obviously incredibly anxious and needs extra reassurance that things will be ok if she leaves. May be unusual but that's not an actual crime, is it?

this

i can understand this. the way guidelines for caring babies changes so quickly, and given this is SIL's first baby, she's bound to be cautious and want to make sure her baby is going to be cared for as she wants it.

my DH and I had our babies a long time ago, and although i've done my research in whats what, my DH goes with the theory that 'i've looked after babies before you know!' and feels the new guidelines don't apply to him!

It wouldn't hurt for you to do that, would reassure her.

People do get funny ideas about their own baby, it's not personal.

Yeah slightly bat shit but my MIL once had my 9mo for the whole day and late in the evening as we were at a wedding and when I got back the new packet of wet wipes in the bag hadn’t been opened. I asked her about it and she said she didn’t use any cos she only did wees and no poo - all day!

She has 4 kids and lots of older grand children so… you never know

She does sound anxious.

Doubt that she'll manage a film & a meal the first time out.

If she really wants to go se might be better off building up to it.

I was like this with my first. Left my mother (who raised me and siblings not to mention already having other grandchildren) a very detailed list of care instructions for the baby while I went out for half an hour. It’s easy to see the crazy looking back but at the time you’re just so in the weeds with it all.

I'd rethink my offer OP because this is a ridiculous ask of a woman who's had three children and when the tasks are so basic. Which indicates to me she is not in the right state of mind to leave her baby with anyone without a need to remote control and micro manage. This will be incredibly difficult for you, and has the potential to end in tears should you make the smallest 'error'. My view is that you either trust someone to babysit or you don't. If you do, other than a heads up about any specific needs of the child, its their watch and they get on with it. If she can't do that, she's needs not to leave him.

You know, I don't think I'd be hugely bothered by this! She's obviously anxious, people do things different ways, babies will be used to different techniques, and you'll probably laugh about it all later. I don't think you should take it remotely personally.

Have to say I feel a bit sorry for this mum as it sounds like she's really anxious about leaving her baby and needs this extra reassurance to take the step. Lots of unkind comments about being batshit, PFB etc. It's not about your competence at all, it's about her anxiety and the fact that she's done it all without help so far. When you're in that position it's really hard to let go and give full control to someone else. Sounds like you want to help facilitate some time off for her and your bro which is lovely so please don't retract the offer. This is what she needs and it's actually a small ask. You know you're perfectly able to care for her baby but right now she's struggling to believe anyone else could do it because nobody ever has. Be kind and humour her!

My brother was living with me when his PFB was born and I made a similar offer. Got told that because I only had sons, they would need to check I knew what I was doing before they would leave their daughter with me and go out. I thanked them for the suggestion, laughed and didn't change a single nappy in the nearly a year they were living with me.

For the best chance for your baby and your cousin I would tell her what you have to do while making it clear you want your baby to adapt so you're happy for her to do whatever she wants but just in case baby objects, I do this.

It does sound a bit bonkers but I doubt it's about you.

Thst said, when I first left ds with my dm I just assumed she knew everything (and she isn't keen on being told how to do anything anyway) and they had a bit of a mare with the nappies as she seemed to have forgotten how they worked.

It's precious first born syndrome. We typed up a semi-serious but actually totally serious manual for ours that my mum had to follow; my mum has 4 children.

I would humour her as most first time mums have had moments like this. If she fails you don't take it personally as she could be looking for a way to pull out and wants you to take the blame. I'd take it. Parenthood can be hard.

Amount of experience doesn’t mean it’s good experience. I have friends with 4+ kids who I wouldn’t trust around my DS with a bargepole and whose own dd’s come to me for child advice and childcare

Inwiudk go with anxiety over leaving baby for the first time...it can do weird things to you. Surely you remember back tomsome of the crazy crap you worried about with your first?!

Be kind, suck it up and smile through it.

I kinda understand from your SILs pov. I have a 6 month old, demanding, only feeds whilst she's lying flat and will only sleep if I'm bouncing (not rocking) in a dark room. I have been really hesitant to leave her with anyone because of these things. In 2 weeks I'm going out and I'm leaving her with my cousin (we are close, she has 3 kids) and a part of me would like her to see how I feed/ put her to sleep just so she has an idea but I won't as I know my baby has to adapt to different techniques so that I can get a BREAK!
The changing nappies one is odd though