To be honest? If Bran decided that he didn't want to be polite then that was fine. Wei Wuxian was used to his baby brother who was normally a perpetual ball of anger. So, it wouldn't bother him if the man before him dropped the act. Why would it? He's been around long enough to not really care. The only way he would is if someone tried to harm him.
"We are 1500 years in the future, so yes," He tells him. "We are all also from Ancient China. So this place is rather... interesting. It took some getting used to." He'd been there first, of course. Then had come his husband, his brother, and then his brother-in-law. Sometimes he forgets that people don't know his native tongue.