"Xavier and Erik could be brothers in the MCU"
even if we somehow DON'T even take into consideration how canonically (b)romantic their relationship is already and therefore how incestuous it would be, it literally doesn't make any fucking sense!
their backgrounds and upbringings being complete opposites are KEY factors to who they are as adults and irrevocably shape their views on how mutants and humans should (if at all) coexist, how to go about it, how to act in certain situations, everything!
Xavier is a rich boy who can have the privilege to think collaboration and respect are always possible cause he always had those! he never truly faced any oppression so he really believes that even when humans are constantly attempting against mutant rights that "they don't know any better, they're just afraid", that mutants should take it on the chin, that they should be the bigger person because somehow, some day, they will just peacefully be accepted! Charle's privilege shapes his worldview significantly.
Erik never had that! Erik is a Jewish man that was thrown in a concentration camp as a child in Nazi Germany! he saw his whole family get murdered! he's seen the worst of humanity first hand, he's seen people treat those who they think are below them for whatever reason they chose worse than animals to the point of killing them! Erik's pain shapes his worldview significantly (he is more than his pain but it's also still part of him).
their opposing views are the result of opposite lives and experiences.
YOU CANNOT GIVE THESE TWO INDIVIDUALS THE SAME LIFE, IT DOES NOT WORK, IT'S STUPID.