Here's a take for Luffy calling Yamato Yamao, she's been portrayed as Oden's parallel and when Oden was with Whitebeard the latter never called him his son, he always refered to Oden as his brother.. I'm not saying Luffy will never address Yamato by her name, he calls people of his generation who he views as the same caliber as him that way, Yamao, Torao.. For instance next time he meets Katakuri he would likely call him Katao or something assuming they are on friendly terms.. If Zoro never became a Nakama and didn't join Luffy's crew, down the line he would have had a similar nickname.. So i think Luffy won't call Yamato Yamato until she joins the crew..