I think you've mistook some details. Anastacious disguised himself to prevent people from recognizing him since he was a bethroned emperor and known as 'dead' to the public. So he is still brother's with Claude since Athy said "uncle" towards him.
Anastacious didn't want to see Lucas because Anastacious knows, if he meets up wih Lucas, Lucas will know that Anastacious is using black magic.
Also I wanted to add that in previous chapters when Claude and Anastacious were kids claudes older brother was seen to be looking at something like a dark spirit ( Aeternitas) near some trees. which can also mean that anastacious has been in contact with aeternitas since he was young by using his black magic. This is what I have understood and have gathered from all the chapters, sorry if I have misspelled any names. ╥﹏╥

So claudes “brother” isn’t really his brother it’s the emperor that Lucas had rejected to teach from previous chapters, I think he just wants to get revenge onto Lucas for rejecting him and to show Lucas how “powerful” he is cause Lucas called him weak. But anyways, I LOVE ATHY, she’s such a bad bitch !