I assumed she was the aunt because:
- They have a small age difference in appearance.
- She confuses the MC with his father (a mother would never confuse her son).
- When you see the memory of the MC's father, the father is older than the MC, so she should know that if his father was older, he should look older, he can't rejuvenate (unless she thinks he's a zombie that resurrected younger).
- She tries to sleep with him (just as a mother recognizes her son, she also wouldn’t try to sleep with her son, at least as long as the visual novel isn’t about incest). (The incest situation with Juli happened because they didn’t recognize each other, and of course, they can continue that relationship, but they didn’t start it because of incest. They even feel guilty, so in the game so far, it's not something normal like in other games).
In the game, it seems like she is the mother of MC and Juli, but to me, it seems more like she is a much younger sister of the MC's father, who was in love with the MC's father when she was younger. When the MC returns, she confuses him with his father because that was the version she remembered the most (although this doesn't completely make sense either). So, when she gets sexo with the MC, she does it because it's a bit more 'allowed' due to the age difference; he is a nephew, somewhat like a cousin.