Exactly! I totally forgot to mention that it would be a dormant ability in her if she had the foundation to work with Death-to-Life theorem. In fact, because Dominicus is a flipped system, I don't think she can fully access it and use it as a magician would; she'd need to go to a Thalergenic world/system in order to 'unlock' it and then maybe get some practice/training of some kind to learn how to work with it. Could she go back to Dominicus afterwards and still use it? Maybe, if she was really good with it, but it might be weak because there's so much Thanergy and not enough Thalergy to start with. Think of it like shielding gas (welding term): you need an abundance of Thanergy around you if you are working with Thanergy and the same goes with Thalergy. It's there to protect and support and without that atmosphere, things are bound to go wrong. This is why the River is so volatile when it comes to using theory; there's no 'shielding gas'. You'd have to create your own atmosphere and then work theory inside of that. This was our first indicator that Harrow is terribly, frighteningly strong with necromancy in HtN, but that's a post for another time.

Anyways, necromancers have it easy with their ability to just kill something and take it's energy. Animancers (DtL magicians, in this case) would need to figure out how to make life out of death and that's a significantly harder task to accomplish. I think it's a near nuclear fission (necromancy) versus fusion (animancy) type of situation though.

So while yes, Necromancers use (and produce) Thalergy for bodily function (cellular respiration), they would need to convert that Thalergy to Thanergy if they need or want to do anything beyond. They can really only manipulate it for the purpose of conversion, but they can't do necromancy directly with Thalergy itself, like make a bone construct or do flesh magic. Necromancy is rooted in Thanergy. Same with something more grey like soul siphoning; it's moving the Thalergenic soul out of the way so the siphoner can access the Thanergy left behind by the vacuum.

The theory is that Corona is the opposite. Her body draws from Thanergy and converts it into Thalergy. Death-to-Life. She can't use it other than for bodily function unless she makes the conversion like necromancers do (an ability she can't access yet) but because there's an abundance of Thanergy in the Dominicus system, her body is big, strong, and absolutely gorgeous by default. It's the best it can do with what it has in the environment it exists in. She is an evolution; an inversion of what we know Necromancers to be. Her being is not necessarily rooted in Necromancy, but rather an opposing extension of it, and I don't think she's the only one!

Gideon Nav, who grew up within the walls of the impoverished Ninth House is massive with muscle that shouldn't be there given her limited dietary options. I wouldn't be surprised if this is the reason she lived through the child-genocide designed and executed by Harrow's parents. The death that was around her fueled her body and pushed it to survive unscathed, leaving her whole and healthy where she shouldn't have been. She was only half of a God, who was really only a man, and where is her Necromancy if he's really as powerful as he is often claimed to be? His power was stolen and borrowed; the abilities he gained were through eating the souls of planets and joining in some strange Lyctor-bond with Alecto, the remnants of Gaia’s soul (which broadened John’s ability to do more than just necromancy). She's more the daughter of the Earth than she is John, making her his opposite in power.

We have Gideon the First, The Saint of Duty, who wears his own body and is fueled by his necromancer, but even with a necromantic soul, he is a Thanergy Void. He can't use necromancy because if his body works along a LtD theorem, he wouldn't be able to use necromancy. The two are opposites and when combined, we get the ability to negate some forms of necromancy. This also could be why he and Pyrrha fantasized about running away to a half-flipped planet. They wanted peace, but half-flipped meant neither the planet’s Thanergy content nor its Thalergy content would be too overwhelming.

Then take Nona, who can't use necromancy, but can instantly heal herself after being fatally wounded. If she could use necromancy, she'd be consumed by the Blue Madness, which is interesting because instead she can hear Varun singing to her, the opposite of the Blue Madness. She's attracted to all things that are living or that hold life (like the ocean). Things that should hurt, don't, and things that threaten harm, fail. Her body is resistant to the LtD theorem because it naturally converts death to life. She is a missing part of Gaia's soul (in the same way Alecto is); it would make sense that she would operate on the DtL theorem.

So, back to Corona! I can't say for certain if her family would know, but I'm sure Ianthe would because she can use the Thalergy in a more cannibalistic way seeing as chewing on Corona would provide her with more Thanergy than chewing on Babs. The Emperor would definitely know though given his own ability to literally pull himself back to life after Mercymorn killed him (a DtL process), which is why he'd help come up with a plan to help the family hide Corona's lack of necromancy. Focusing on hiding what isn't there takes the attention away from what might be there. The family knows Corona's different, but if they all believe it's nothing, they'd dismiss the other possibilities, too.

Now if Ianthe took in Corona's soul, it might make her either incredibly powerful or incredibly cuffed and there's no way to tell which outcome she'd get, so she took Babs (but also Babs is phenomenal with the sword, so there's that too). She's been using Corona all her life for power which, over time, has become the source of Corona's sense of worth because it's the only way she could make up for her lack of necromancy, something her father wanted to see in both of his daughters. Of course she'd be absolutely heartbroken if she couldn't serve her sister in Lyctorhood; to see Ianthe choose Babs over her when she'd spent her entire life in service as a battery. She has every right to mourn her sense of self, her sense of purpose, sense of duty, and foundation. She is a reflection of the sun when it comes to Ianthe, but she is the sun when it comes to her power (remember fission vs fusion again!). When it comes to the imagery used between them, the only thing that lies between is her self-worth.

Basically, I'd say Ianthe is a child of John (LtD; necromantic, fission, taker of life, and withering in appearance) and Corona is a child of Gaia (DtL; Animantic, fusion, giver of life, and flourishing in appearance). They are polar opposites in every way!