Oh god, I really didn't want to get into the back story, as it's really the ethics of the "sharing correspondence" I wanted advice on.
I didn't know until recently that I had an antagonistic relationship with SIL!
I've always assumed that if DB rated SIL, she must be a good 'un.
They live far away, are busy people with exciting lives, and I'm disabled and can't easily travel, and try my best not to impose on other people. So I barely know SIL because I don't really see her, and don't usually hear from them from one year's end to the next.
I usually send the children presents, and get thank-you cards from SIL, or an Xmas card in my DB's handwriting, but that's it. Occasionally they've had family news, good or bad, and I've sent cards and emails, and been congratulatory or supportive, but they don't call or initiate contact so I'm not a big part of their lives.
They have a bit of a history of expecting me to run around after them to visit them (in wheelchair with luggage on public transport, with me booking hotels & taxis to break the journey as I can't do it in one), and I've always taken this in good heart and assumed they were just too busy.
They also have a bit of a history of cancelling me at the last minute for a subsequent engagement. Again, though it's hurt, I've taken it on the chin and cheerily made the arrangements for the next time.
Last year they excelled themselves in this area, and at the same time SIL picked a fight out of the blue about nothing (she wanted to "help" pack my baggage: she told me afterwards she needs to feel useful. I had unsuspectingly very politely declined as I need it packed a particular way, but had asked her to do lots of other things I genuinely needed done). She laid into me the next morning about it too.
A lot about the way she did it was extremely familiar, and left me staring at her horrified, thinking "Dear god, DB has married his mother!" (Our not-so-DM is a professional victim of any circumstance ever and can emotionally blackmail for England.)
As DB & SIL live far away – indeed have now moved country – there was no pressing need to deal with this and I just let it all drop. Sent nice email to my brother, etc etc.
But it dawned on me (thank you MN for opening my eyes to this dynamic) that the more I ran round after them, the less they valued it and the more they expected.
I'm really very unwell and have very little useable time each day. I actually need help around the house and can't live fully independently. So I decided I wouldn't be dedicating the tiny bit of time and energy I do have, to running after them.
So I haven't.
Given they so rarely initiate contact with me, I just haven't initiated any with them. So... we haven't emailed for a while. Also the children (who are too young to know or care) didn't get birthday presents.
For anyone saying, "There must be more to it than that, for SIL to lay into you," all I can say is, "Not as far as I know."