I'm thinking of a story idea but I am not sure if the concept of reincarnation (i.e., someone being born into another life and having only hazy suggestions of the past life) exists in Lusternia, and I don't know if I want to invent it for this.

I believe so, but not so much with remembering past lives I think. People have a hard enough time remembering their own lives before the portal

Absolutely, and in fact kephera queens are reincarnated many, many times - and the way you "decide" it makes it unclear whether it's a mechanical thing that forces it to be the case that whatever kephera queen is needed at hand happens to be reincarnated into whatever female hatchling you have on hand, or if you're supposed to believe it's a coincidence. Their memories are initially weak but once you bring them banners with their king's scent they remember who they are. Zenobia has the same thing going on only she also has to drink a lot of potion and smackdown all the others.

Either way you're not inventing anything new.

Ecology is highly influenced by reincarnation. See: Rebirth

This reminds me of that girl who thinks her past life was as an Ackleberrian.

When you die, the sequence says that you become aware of the many past lives, or some such. So yes, you could easily put reincarnation into a story.

I shall make the argument that it is destiny that you picked that King, got that flag - not coincidence. DESTINY. You know, it's not like Lachesis is sitting around on her thumbs doing nothing...