God as much as I wish I could agree with you, I can't. I have seen middle school and Highschool girls do the pettiest things, and have even seen guy friend groups make mountains out of mole hills. I had a friends that were cut out of another friend group because they didn't agree with something...
So while I do think that she is trying to achieve is not a good basis for a relationship. Your comments are actually wrong. Love is a not forever feeling, and it is replaced by companionship. Expecting love to last forever isn't realistic there are also different types of love.
My overall...
I still don't think it qualifies as incest at all because, she was adopted, this was after the older brother disappeared, and its already implied that they are reincarnated lovers and lily just ended up at his family's house. Like if he had met Lily as a sibling even once. I would agree, but...
I don't feel bad for the reincarnator in Pina's body. I feel bad for Pina who most definitely stuck watching her life be derailed and taken advantage of by a rotten person.
Right its not even like this is one of those on fire dumpster trash manga, its just a freaky friday romance novel with a love triangle. Little to no drama. The biggest drama is the fact lemon lied about cheating to the mc.
But the FeMC did something bad and therefore everything she does needs...