NC for this.
Long story short, my sister and my ex BIL divorced around 6 years ago. All water under the bridge now, they don't have contact anymore, no kids together so a relatively straightforward separation. It was hard at the time as BIL had been a part of the family for a lot of years and I regarded him as more of a brother to be honest, so the separation was hard for the rest of the family but it was the right decision for my sister and we obviously stood by her as her family. I'm still friends with BIL on social media but no longer close, and in all honesty we have very little contact. I messaged him after his child was born to a new woman a couple years ago to congratulate him, and he replied thank you etc, but otherwise we don't generally speak.
Today I received a message from him out of the blue asking how I was. I replied that I was fine and asked about him etc, we had a pleasant exchange about our respective kids etc. All fine and pleasant. He then asked me if I knew why my sister was still using "his name", and that he'd discovered this recently and it bothered him. I said I didn't know why she had made this decision, and that it's not really anything to do with me, but I assumed it was because it was less hassle to keep her married name for practical reasons (my sister is a very pragmatic person). I explained this was likely the reason and he continued to state that this was "weird" and "wrong" and could I "suggest" to her that she should revert back to her maiden name. I said this wasn't my place and legally it's her name, and asked why it was a concern to him so many years down the line? He said that I wouldn't understand because I "hadn't been through it", but that this surname was "a part of him" and he didn't need the "constant reminder". Again, I asked how on earth he's being constantly reminded when he literally has no contact with my sister anymore? He couldn't answer this, just said "it's not her identity anymore, it's MY surname". I was going round in circles explaining to him that it was also in fact her legal surname to use as she wished and there was little he could do about that. He then stated that "there is only going to be one Mrs X soon and it's not her". I pointed out that there are many, many Mrs X's in the world and he didn't own the rights to that particular surname. I suggested he just focused on his new relationship/ wife to be and family and move on from what my sister chooses to do / not do. He then became defensive stating that "I don't care about her or what she does but I don't think this is right". I had to end the discussion as we were just going round in unhelpful circles and I was fed up of repeating myself.
I then spoke to my sister and she said he'd also been in touch with her recently out of the blue saying similar things, demanding to know why she wasn't changing her name. She said she told him she'd made a decision to keep the name she was known by in professional circles and just for ease. He was apparently unpleasant towards her so she ended up blocking him, hence he's now badgering her family members.
AIBU that this is just a ridiculous thing to for him to get wound up about? Why now, 6 years down the line is my sister's name suddenly an issue just because he's remarrying?! He surely cannot expect my sister to change her name just because he finds it "weird" and thinks its "not her identity"?! And also why is he bothering me with this?! I assume this has only become an issue because he's getting married again, but why should it matter? For context the surname is a very common one - think Smith / Jones - and so there'll be literally thousands of women with this name across the world and yet his new wife needs to be "the only one"?!
Aside from all of this, what has any of it got to to with me?! I wasn't even married to the guy!
The whole thing has just annoyed and baffled me and just wanted some outside perspectives! Thank you.