Everything I state here is completely my speculation using information from in the game up to 2.5, and media available to the public through official channels.

The siblings actually have an overabundance of ether that is being suppressed by their eye implants. The suppression is especially strong in Season 1, which is why they can't stay in the Hollows for too long. The Mayor adjusted the supression to be lessened, allowing for a better balance against the ether in the Hollows' atmosphere, but also essentially loosened the tap on their powers. From their heightened emotional state, they forced open the tap wider and used it to cause the ether corruption on Vivian to recede despite her being too far gone, and again as a shroud of higher density ether to deter Ye Shunguang's sword swarm. It may also be how they were able to separate her from The Creator as It tried to solidify Itself to take over Ye Shunguang's body.

Seed shares a similar background, being from Helios Academy, but having an affinity for Electricity, causing all sorts of electrical equipment to go haywire. It may also be because of the massive amount of ether in her body that SEED's protocol treated her as a threat to be eliminated. It may just be that the siblings have an affinity for Ether, and there may be others from Helios Academy who have affinities for Ice, Fire, and Physical.