immortal does not mean invincible...
i mean, with strange aeons even death may die...
I blame a combination of Chinese Wuxia works and vampire fiction for this, that throw around the term immortal like dollar bills at a cheap strip club.
Immortal literally means the opposite of mortal, mortality signifies one's inevitable death. Hence adding the preposition "Im" makes it the lack of death. Unfortunately the word has long since been water down, but an immortal is literally invincible and ageless.
Someone who cannot die of old age, but can die from other means is Ageless. Typical vampire.
Someone who cannot die from anything other than aging is Invincible. Closest example I can think of something like the Hulk.
Someone who is immortal is both. Its literally the definition of the word. Yog-soth/Azatoth is an example. Closest anime example I can think of is the little mini guy in Dragonball-verse with the blue and purple face.
For the record Immortals in most Wuxia, are typically none of these. Ive read works where they barely live longer than a regular person, and they are almost never depicted as invincible. I suspect the Chinese use some different word like taoist/sennin that just gets mistranslated into immortal and people have since run with it.
For the record another common set of words that get butchered in Media are the Omni-words, Omniscient, Omnipresent, Omnipotent. These literally mean all knowing, all existing, and all powerful. So no unless your character is literally the Level 0 god of your verse, you are likely not any of these terms.
But have to agree with the others on here, I don't mind the sisters as much, but this arc with the demon lady has felt very slow. I don't mind her as a character, but this should have wrapped a chapter or two ago.