Personally i have a huge problem to think Ellie and Mia might have only 1 year between them. The size, the experience and the way Ellie talks "Stupid Uncle" "Stupid Mia" fits more to age 10-12, the whole behavior don't fit to 1 year difference especially with 2 girls living alone with a single parent. It's normal that the younger imitates and try things, the older does. In Ellie and Mias case both seem like they have no connection outside school and absolutely nothing in common. That doesn't work very well at all with a sister theory.
IF Ellie was MCs daughter, Olivia would have said it the first private moment to seal the connection to MC forever. She also seems to have no problem about a crush of her daughter to her as someone thinks, father, the MC. Which mother wouldn't interrupt that, especially if this person would be the real father and he didn't know it or if he know it.
Ellie is searching for a father figure. So she gets used to call the actually man, who lives with them in a household and who is very kind to her and cares for her, daddy. If she would have sensed, MC is her real father why still talking about Lucas as daddy. And if she sensed MC is her father, how absolutely corrupted would her whole live already should have been to make out instantly with her father. Ellie is the inexperienced, kind daughter, who cooks and looks after older "sister" and her mother, but then you think the first thing she does with her sensed father is making out with him and working on his dick? Unreliable theory and if true a really bad story.
Olivia had a hard time with Mia as a baby with a messed up life and she actually lost her best friend and her man she loves. Lucas had the money, he cared and he had a codex connection to MC to do everything for his daughter and so also for Olivia. But at some point both had only the memory of MC in a hard world. One thing leads to another and friends land in bed. Nothing fancy but absolutely believable. And it wouldn't be the first kid born through such a situation.
The clear flashback of a 1 years old baby about her dad and Olivia and her real mother.... Man that was absolutely over the top. The earliest memories i can truely remember were from age 3 or 4. That are more like shadows of pictures, memory snippets.
Before that time, i only have a vague feeling to be deeply loved from my mother and that we lived alone back then. I also have a single more complex memory scenery, when I laid in a buggy through noon. I can remember the wasps, the sweet smell of pears in the backyard, where my mother and me lived alone in household. Someone hung out the laundry. It is like a scene who fades away the moment my brain tries to grab the picture. To be honest, i might think and i hope this will be the last thing i remember before i die. It's a priceless memory.
But Mia should have clear flashbacks of 2 scenes with 3 different persons? Wow, mastermind, genius, that was too much for me.