I’m struggling with my relationship with my SIL (DH’s brother’s wife) and hoping for some advice.
She’s always been a very difficult character (glass half empty, always very negative, very insecure, can’t be happy for anyone, thinks the world is out to get her, rude and verbally aggressive to anyone and everyone). We have as little to do with her as we can, so only see her at wider family events.
She had a baby girl a couple of months ago and has struggled a bit with things (just the normal lack of sleep, getting used to being a mum etc, as well as general Covid worries).
We’ve been as supportive as we can (sending gifts, nice messages, a bit of money when they said they were struggling, etc.) but haven’t seen them as we haven’t been invited and we haven’t wanted to impose by just showing up.
Last night she sent my DH and I a really nasty message accusing us of not being supportive. She didn’t really specify anything we had or hadn’t done but did include a random accusation about us forcing her to come to our house (we have no idea what she’s on about as we haven’t asked her to do that at all!).
We were quite upset by it and DH rang his brother to find out what was going on. His brother is a man of few words but said something like “yeah mate well you haven’t really helped us out since our daughter was born, so you could’ve shown more support”. DH was speechless as we’ve definitely shown a normal level of support and we’re not close to them anyway so it would’ve been inappropriate for us to go round and start muscling in with things with their DD. Knowing what SIL is like, whatever we would have done would have been wrong, so if we had gone round she would have sent a nasty message saying we were intruding on their space etc.
We’re really ground down by how she is and wondering where to go from here. She doesn’t normally lash out randomly like this - it’s normally lots of digs when we see her, so we just keep seeing her down to a minimum and do the whole “grey rock” thing as much as we can / spend most of the time at family events with other relatives. But now she’s lashed out I’m wondering how to respond.
Personally I want to respond sympathising with her difficulties but then defending DH and I, setting out the things we’ve done for them and then asking why she felt the need to lash out like that.
DH’s sister thinks we should cut her some slack as she’s recently given birth (8 weeks ago) so should respond very nicely, apologising, even though we honestly haven’t done anything so there’s nothing to apologise for! I also know many others who have given birth and none have acted like this at all.
DH thinks we shouldn’t respond at all as it will just “feed the beast”.
I’m thinking whatever we do will set the tone for the next few years while she’s a “new mother” so I want to get this right. My worry if we send a nice message without defending ourselves is that she’ll then think she’s entitled to send more of these aggressive, nasty texts and we’ll have set the tone of putting up with them. 
Do new mums get to act as they wish, no matter who they hurt? Any advice would be very appreciated!