It is not mentioned in writing that Amelia and Lana are about the same age. But when I look at Amelia, Lana and Jill and compare them, Lana is much older than Jill, Amelia looks like a normal old mother for Jill's age.
Now if Lana were also Amelia's daughter, Amelia would have to be at least 37 years old at the time of the flashback, I'm now taking your estimated age of Jill (10), because the parents decided that Lana should and/or must go. Then Lana would already be 18 years old at that time. It goes further. In EP8 at the concert we see Amelia. She still looks good for a woman of 48. Sarcasm.
Why 48 years? Jill is around 21, ergo 11 years have passed since then, for your assumption that Lana is Amelia's daughter, add Amelia from the flashback to Amelia from the present. As I hope you can see when you look at the characters' faces, my assumption is not that improbable.
It's fine for you to rely on what is written in the game. But it is not written anywhere that Lana is Amelia's daughter. The only thing we know is that Lana is Jill's sister. (sister, half-sister, stepsister).
