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.