So I dont really have an answer for this, but I do have speculation. Her nephew's birth killed her sister, someone she was super close to. He also didn't resemble her sister at all and instead looked like the bastard that abandoned her. To vent her frustration at suddenly losing her sister and being left with a kid who looks like a guy she hates, she would mistreat him. Its not a great reason to hurt him, but it makes sense.
For the spoilers that I have read, it seems that she doesn't remember her first life? I don't know, if reincarnation really existed, and one of my past lives did something horrible, I would not feel responsible for their doing, as I am a different person even if in theory we share the same soul.
The only things I have to say about it is, it seems unnecessary, like, does it serve a purpose that she is leaving her 3rd life? And, well, even if that's the case, she already paid for her sins by being killed, so fair enough I guess.
Yeah, and I know abuse shouldn’t be justified but the reason she was able to reincarnate as well the third time like this was because Luca eventually forgave(or something like that) her because he thought it was understandable how aunt eventually felt towards him. And even then, the reason why she died in the first life anyway was because in the end she ultimately wanted to save Luca from the actual villain when she found out the villain wanted to kill Luca.

What do you think that a mature FL was a child abuser or basically mistreated the protagonist most of the time in the first original timeline?