I was just reading Wolverine and the X-men 26 and 27 and remembered the questionHow to Kill Wolverine? My question stems from the fact that in mutants siblings' mutations cannot harm each other, for example Havoc and Cyclops. Would Wolverine's healing factor come into play if he and Dog would fight to the death?
Mutant sibling power exceptions are very difficult to document, since there are very few active mutant siblings from which to base a hypothesis on. Given such a small sample of mutants to draw from, it cannot be accurately assessed that siblings will always possess abilities which will allow them to resist, reduce or absorb the power of their sibling mutant.
I think in the case of the Summers brothers, their powers were all energy related and this gave them some limited ability to absorb, reduce or refract that energy away from themselves since it resembled an energy they were already familiar with.**
The only commonly seen active pairs of mutant siblings include:
Charles Xavier/Cassandra Nova
The only one of these which exhibit "sibling power exceptions" are the Summers brothers, who are all able to reduce or ignore damage from the energy projection aspects of their powers against each other.
The Scarlet Witch's powers of probability manipulation work quite well against her brother and at some point against all of the Avengers and mutants world-wide. Pietro had no particular resistance to any of her powers, nor she to his.
Northstar and Aurora had the same powers initially though over the years with genetic tampering their powers had been altered. Both possessed the ability fly (theoretically able to approach the speed of light but this has never been proven since their mass did not grow less as they did, creating catastrophic environmental effects. Their powers offered a third complementary ability when they touched they could emit a powerful burst of light equal to one million candela. With recent genetic tampering they can now emit a thermal burst as well.
Xavier and his clone sibling Cassandra Nova were equally able to affect each other with their vast telepathic abilities, though Nova's adaptation of Xavier's genome gave her access to latent powers that Charles Xavier did not appear to possess. Her possession of those latent abilities allowed her to defeat Xavier and for a time take over and trade his body for hers.