My brother's fiancé sent me a save the date addressed to the future mr. And Mrs. his first and last name. My brother knows I'm keeping my last name. He has talked to me about it, encouraged my decision, and even noticed/commented on the carefully crafted return address labels I sent (pre covid wedding date/invite) with both of our first and last names. So I honestly think she did this on purpose and I'm livid. This isn't my mom's friend making an assumption when buying us a gift with his name (which was sweet and sits on his desk now), this is someone who knew and still put the wrong name.
(Background: I'm currently not speaking to either of them because of hurtful things they have each said to me/my family about the importance of their wedding compared to mine. The weddings are close together. Working on forgiving, so I don't want to go more into it)