Shiki stares at her cousin.

Kill things that don't have a physical form?

"I ' don't understand what you mean."

The red lines that she sees appear on everything in her surroundings. Her

physical surroundings,

in the world around her. These lines symbolize

death,

so cutting a target's lines will 'kill' it. That's essentially the long and short of Shiki's ability.

Even the cursed techniques that she's able to kill are things that

exist

and are

present

in a physical, literal sense. Which is likely the reason why she's able to see the lines of active techniques at all.