Kaeya is sentimental for his childhood, he is seen looking longingly at the winery in one cutscene and in a birthday letter the wind took him there. Knowing you have no power to stop Diluc selling the mansion as you’re not the blood inheritor, must hurt because that’s his farther stuff too 😢
I think he has a right to feel upset, they definitely grew up in both places. He would have seen it as his home. He said to Diluc something like, will you throw me out with the trash? same way you threw out father’s legacy, you sold off his mansion. Must have hurt a lot to watch Diluc erase it all.