SIL has stopped me seeing nephew

[Magmum13]

My nephew is 3 years old. My brother's partner has displayed some coercive controlling behaviours in his relationship, they split up regularly and she's often involved in conflicts with her family and sometimes ours too. I have managed so far to maintain a relatively good relationship with her. My nephew means the world to me, I see him and mind him regularly and have done since he was a baby including having him stay over with me.
My SIL has picked a fight with me, I should have seen it coming really and I'm cross with myself. It was over some wording in a message about a party invite. Very trivial. She thinks I was rude, I don't. She said some quite unpleasant things about me and blocked me everywhere so I can't reply. She's then asked my brother to get involved. I asked him not to and reiterated that to resolve the argument would need for her and I do agree to disagree and move on. In what I thought were private messages with him, I have explained why I think the enforcing of one party to capitulate in disagreements is a facet of control, I have also explained that her behaviour isn't in keeping with someone who wants a resolution and that instead my hunch is that he is being positioned to choose between us. She's then read those, and sent a rather explosive message to me saying that I am unhinged and dangerous and must therefore be kept away from my nephew. I've spoken to my brother who acknowledges that there's no truth in the suggestion that my nephew would be somehow at risk around me but considers himself in a difficult situation where he must prioritise the stability of his relationship with her. At a family get together this weekend where he would normally come and bring his son he didn't turn up. Other family members are reluctant to get involved as they say they recognise it will be them next and nobody wants to lose contact themselves,
I feel there is not much I can do here, and I'm not sure what kind of advice I'm looking for, rather it would be good to hear from anyone in similar circumstances and feel not alone with it. But if there is any advice at all I would be grateful. It breaks my heart not seeing him.

What a reach. You seem to be a dramatist not the OP!

Yes this. My brothers wife did the same to our family. She declared that as the mother it was up to her who saw the kids and systematically fell out with my parents, other brother and myself. We haven’t seen our brother or the children for many years and, to be honest, wouldn’t knkw them if we passed them in the street. My brother made his bed and chose her poison over his own family so 🤷‍♀️. Let it go. You’ll never win ans she’ll always use your need for a relationship with your nephew as a way to control you and get you to do what she wants. Just hope your brother finds his backbone one day and leaves. Mine hasn’t so far, but his loss.

Sorry OP, but your post about what exactly happened made me change my mind. Instead of replying ‘sorry SIL, we can’t come as it’s my birthday weekend - is there any way the date could be changed?’, you sent a pretty cryptic reply that screamed ‘me me me’. And yes, people do arrange Christmas parties in November because December is usually such a busy time. Our wider family Christmas get together is in November!
However, you’ve decided as someone with a degree in psychotherapy that she’s in the wrong for being angry at your scathing messages to your brother about her which has resulted in her not letting you see her son. Now you’re so sure that you’re right and she’s wrong that you’d prefer to stick to your guns rather than apologise for upsetting her so much so that you’re willing to have no relationship with your nephew. Not only that, you’re pinning the blame for that on her!
Absolutely lacking in self reflection here.

I have a similar situation. SIL very controlling. As such haven’t seen my brother for over 5 years.
hes been turned against me and my family and she makes sure he doesn’t attend anything we invite him to such as christenings etc. She’s a nasty person who has a lot of jealousy and hatred inside her and she only feels good controlling him. She sees only her family of course.
I've give up trying now. My brother will always support her and as such we don’t see my nephew and my children don’t know him.
some people are just psychos. She of course tried to frame me as one like yours did.
my advice is to leave some space. Maybe invite them to something as a peace offering and see what happens.

I think it was misguided of you to message like that and be unreasonable about her party. To then send that message to her dh was probably the last straw. Whether true or not, it's disrespectful.

I don't think people are generally keen to leave their small dc with people who don't like them. It's about trust.