To be annoyed SIL showed DS her lactating breast

If my SIL did this, I would not like her.

Seeing a nipple isn’t the issue here

I think if the OP actually liked her SIL this thread and this whole discussion wouldn't exist...

Yes a bit concerned at the idea you can and should breastfeed without anyone seeing a nipple this in my experience was impossible and very quickly let go the idea that I should protect people from the nipple.Generally think its a good think that a child would learn that a nipple in the context of breastfeeding is non sensational.

We're working with a voicemail from the SIL who was quite likely summarising rather than repeating the encounter verbatim, which has then been recounted here by an OP who doesn't like the SIL and has form for exaggerating ('squeezing her nipple into his face') and drip feeding.

I'd caution against getting too invested or assuming the OP's version is an objective one.

I'm currently breastfeeding and would find this a very inappropriate thing to do. I breastfeed around my nieces and nephews, but keep my boob and especially nipple very hidden from them. No kid needs to see that!

All OP says is that her son walked in on her SIL. Nothing about SIL wanting privacy. Maybe she'd moved to a different room to find a comfortable chair.

Very weird.

Kids are sometimes encouraged to squeeze a cows teat at pet farms to understand the concept of producing milk. When you think about it it's bizarre that children are encouraged to see a cow do something that humans do too but we are uncomfortable with our own species. It's something I would never do and would be mortified but I actually think she is right and children should know what breastfeeding is.

Then she would have been expressing. But she wasn’t. She chose to squeeze her nipple for her nephew to watch.

Perhaps the OP or her DH should be supervising their child then, instead of letting them wander into rooms where breast feeding mums have gone for privacy. What if the mum had been in there expressing?

@Runningswanker "SIL hasn't gone out of her way to educate him, she's responded in the moment when he's walked in and seen her."

But she did go out of her way! The boy walked in and sees her breastfeeding. He asks what she's doing and she says "feeding the baby". No problem. But she then shows him her nipple and squeezes milk out of it. The boy didn't ask to see that and her original reply was quite sufficient without checking with his mum.

You are massively overthinking. It's normal and natural and she did the right thing treating it as such. He will grow into a better man because of it imo.

Teaching moment to a suitably aged child.

No I don’t think that’s it. I’m pretty sure the nipple squeezing one is the oddball.

And if the OP already dislikes her and doesn't trust her, why was her four year old in a position to wander in on her in the first place, when the OP presumably isn't there (given that the SIL told her by text)
IMO normal functioning people dont spend their time gossiping and being critical of what others are doing though, so perhaps your view of normal and functioning isn't as widely accepted as you think.

She responded oddly. Some people are strange like that. Normal functioning people would manage to navigate that situation without squeezing their nipple. She sounds like one of those where you’re constantly rolling your eyes and saying “god you’ll never believe what Jean’s gone and done now”

Re communicating it to her maybe something like “No issue with you breastfeeding in our house and DS seeing it but please don’t give a visual demonstration again. If he has any questions I’m happy to give him a verbal explanation”

That would be a strange conversation. I just cannot imagine raising that with my SIL! IF my young son had been asking aunt such questions then I'd steer him back to his parents.

It is not, but breasts are.

When we talk to our boys (and girls) about breasts I'm pretty sure we're not just talking about BF, but instead talk about both functions.

It's weird that you think breastfeeding is sexual.

It isn't sexual, so doesn't need to be hidden from young dc.

Yes - I’m sure I must have received similar demos! Lots of younger siblings/cousins, lots of medics in the family….everyone pretty matter of fact about breast functionality!

@Livingonbananabread "I regularly saw my aunts/my mum breastfeeding when I was growing up "

But the OP was fine with the breastfeeding bit. She didn't like her SIL giving her son a demonstration. In other words showing him her nipple and squeezing milk out of it.

I don’t think it’s a big deal, and wouldn’t have occurred to me in the SiL’s position that an explanatory voice note was remotely necessary. I regularly saw my aunts/my mum breastfeeding when I was growing up - as well as getting changed/in the bath etc. and would be similarly unconcerned by nudity around my own nieces and nephews, or my children seeing my siblings.

What I do think is telling is the way that people who don’t like the sound of the behaviour have distanced the aunt’s relationship with the child by calling her “not a blood relation” or “an aunt by marriage”. The OP clearly said it was her husband’s sister, so a close blood relation to the child. Just because she’s only related by marriage to the OP doesn’t diminish her connection to her own nephew.