Commenting in case someone spoils it (⌒▽⌒) But at this point it really wouldn’t matter even if his identity was revealed. He’s given up all association with the orthodox sect, his path is only getting further away from them. 1.they’re pretty weak 2.he’s taken all the necessary cultivation books/items from them. But i do believe a reveal like that would have the most impact if it was targeted at the other brother. If he said something like “You trusted me so much because of our blood ties but i’m not even your brother” ლ(´ڡ`ლ) because that poor dude is so conflicted with his love-hate feelings for the mc
Spoiler Alert
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Does he ever come back to the house of the guy he is pretending to be? The answer is NO. Actually it's like they're forgotten lol.
Does his pretending ever get found out by anyone? Yes and No. He only revealed his identity to someone he trusts which is barely few... Throughout the story he just went on with his identity as Mok Gyeong Un and later on as Cheonma...
PS. I have not finished the entire novel just as yet but I'm way advanced from the manhwa. So... If you really want to know, read the novel. It's way better since there are scenes that are in the novel that's not really included or elaborated here in the manhwa.

Does he ever come back to the house of the guy he is pretending to be? Does his pretending ever get found out by anyone? I like reveals so much