>>127263656Mastermind is in some of Claremont's early 80s stories, but once he commits to trying to redeem Magneto, characters from the original Brotherhood just don't appear in his run, so they can't remind readers who Magneto really is and what he did. Blob is the only exception to this, but he didn't last long on the team compared to the others.
Byrne, Stern and some other guys were the ones who decided they were going to make Magneto into the father of Wanda and Pietro, according to Byrne, Claremont didn't even know it was happening until he saw the page Byrne had drawn setting it up. Either Claremont never had any interest in the twins as characters at all, or he just knew he couldn't do anything with them without making Magneto look awful. In his entire 1975-1991 run, Pietro never appeared, Wanda only had a minor appearance as part of the Avengers team in the Kulan Gath story.