Michael Spinks by UD in 12 rounds I think Leon was more of a heavyweight (strong and powerful), while Michael was a natural tall light heavyweight. Despite this, "Neon Leon" lasted 3 rounds with Larry Holmes, while Michael lasted 30. Setting aside the fact that neither of them would want to hurt each other, a KO from Leon is highly unlikely.
Michael Spinks jinxes him, far more versatile, Leon only has one way, which is forward, Michael could step back, measure opponents and find the gaps and flaws and capitalise, he could set you up for power punches, Let’s n just relied on his awkwardness and his engine, Michael would walk him into big punches consistently and would knock him out.
Michael was the more skilled, his way of fighting would have worked against Leon's. I can see M stopping him late.
Michael Spinks was more talented and overall a better fighter than his elder brother, Leon. Therefore it only makes sense to favor him. But there’s a huge caveat here when dealing with the psychology of brothers facing each other which could lead to an unexpected outcome.
I'm going to go against the grain here and say Leon takes it. The bigger brother always wins. It's nature.
That's precisely why I favor Michael Spinks. Leon's style was more focused on harming the opponent, whereas Michael's style was more defensive. Leon, being the older brother, wouldn't want to hurt the younger one, which would only increase Michael's chances of winning by decision.
One question for me is: How much better was prime Michael Spinks than the version of Muhammad Ali, who fought Leon Spinks? I suspect Michael was not better. This was not quite the severely diminished Ali, who fought Larry Holmes or Trevor Berbick 2 to 3 years later. Muhammad and Leon split 2 fights so basically went even. Michael Spinks was a better all around fighter than Leon Spinks. BUT. I believe the styles actually favor Leon Spinks, which somewhat negates Michael's other advantages. And, I agree that neither would be out for blood against the other brother. I am going to say the various factors involved even out, so I will go with a 12 round Draw. No sister kiss but the two brothers can hug it out afterwards.
Michael said that Leon used to beat him up in the gym until one day Michael beat him up. Idk how often they kept sparring after that.