I'm not sure if this is an AIBU but let's see.
I'll try not to drip feed while keeping this brief.
Later tonight I'm having a virtual wine with my best friend from uni. She is a great girl but bless her, can't keep a secret.
During lockdown 1 she became close to my SiL (DH's sister) they were bridesmaids together for me so have known each other a while.
I'll be first to admit I've been a bad friend this year. I got covid at the start of the last lockdown and it's taken until now to get back to normal. This lockdown I'm making more of an effort to keep in touch with friends etc.
Anyway, tonight is the first time I've properly spoken to this friend since I got sick (we've texted etc but finding time / energy for a full on video call hasn't happened)
Over the year SiL has become more and more selfish and has done a lot to really upset her brother and me. At the moment we are avoiding her as we know it's a tinderbox about to explode. We have everything crossed we don't have to see her at Christmas. He is so angry at her but doesn't want to start a fight at the moment.
I know friend will ask me about SiL tonight and I will try to avoid the topic... but ...
Would it be totally unreasonable to vent to my friend about how awful SiL has been knowing full well it will get back to SiL?