I don't know if this is a bug or not, as I can't see it in the changelog. I can't seem to get back in a relationship with the red sister. I remember you get a scene where you can tell her you love both sisters and talk about it, but it doesn't pop up anymore after replaying several routes/answers. I always end the harem without her.
Viewing post in Tales From The Unending Void: Season 3 (v0.24) comments
That was the weird part, I was in a relationship with Lilly in season two but kept losing her when proposing the harem with Celine and thought I had to make it up later in the story.
But I finally found it. When Lilly finds out about Celine I always continued with both of them. But I had to then tell her about the others. My mistake.