Long story - sorry!
DH and I have been married 13 years. His brother got divorced around the same time DH and first wife split up (around 15 years ago). MIL still has large pictures of both son’s weddings in her lounge. It has never bothered me - she also has smaller pictures up of our wedding and all the grandchildren. But BIL is now getting married and future SIL has told me that she finds it really weird and a bit upsetting. She has asked me to support her in speaking to MIL. I’m torn - I can see why she finds it weird after all this time but it’s MILs house and she should be allowed to have whatever pictures she likes. I get on really well with new SIL and want her to feel welcome but I can’t honestly say it bothers me.
So
YABU - SIL is wrong, stay out of it
YANBU - SIL is right, it’s weird.