I know Havok has a limited degree of energy absorption powers, but Cyclops doesn't and he's still immune to Havok's abilities.
Havok has slighty more than
A limited degree of energy absorption
Havok is constantly absorbing cosmic radiation and can absorb or drain specific energy targets.
Cyclops also does this but to a lesser degree, absorbing sunlight to charge his optic beams. He has also absorbed lightning from Storm though this caused him pain. - Uncanny Xmen Annual #3
Neither Cyclops nor Havok are fully immune to the powers of the third Summers brother Vulcan. As Vulcan is an Omega level mutant this could just be a case of his powers being too much for the others to absorb.
Interestingly Vulcan is immune to Cyclops' powers and can even control his optical blasts, - X-Men: Deadly Genesis #4
But Vulcan is not immune to Havok's powers, which would suggest that the energy Havok releases is more powerful than Cyclops' energy.
I would say that the Summers brothers can cancel out each others powers (mostly) due to the fact that they can all emit and absorb energy.
All the Summers' brothers powers work on an "exotic" type of energy, with Cyclop's being [arguably] the most so, in a "pure" manner. I say this because we can definitively site the source of Havoc's powers [ambient cosmic radiation], and Vulcan's powers aren't energy based per se, but psionic; he can mentally control energy on a whole, as opposed to pulling from a specific source, making his more general. Originally, it was thought [even by Scott himself] that sunlight "fuels" his power, which it turns out is untrue; sunlight instead seems to feed his body's capacity to unleash or safely tap into his power, which is actually an unknown energy which stems from *a whole other dimension.** This energy gives off a ruby-colored light, but no heat, no induction, no radio waves, no electricity.... nothing but an intense form of concussive force.
This would put this energy squarely outside the electromagnetic spectrum, and most known forms of radiation. The other Summers' brothers tap into those other energies, and Vulcan can even tap into magic. However, this energy being the most "exotic" arguably means its also the most limited, which is why [generally speaking] Havoc's purely destructive power often appears to be more widespread than his brother's, and why Vulcan's is on a whole other level. This would also explain Vulcan's ability to manipulate mutant powers; on a base level, he's manipulating the energy produced from the mutant power itself, the energy within the body, or the electrical impulses in the mutant brain which triggers the ability.
All of this was necessary in order to fully understand the brothers' "so called" immunity to each other's power.
It was explained that each Summers' brother is unaffected by their own power by generating a "natural energy field" which essentially negates the affect of that power on the tissues of the body. Considering the sheer volume of energy Vulcan can utilize, as well as the differing types, its unsurprising that he can"overwhelm" this natural defense and cause pain or damage with their brother' powers.