Playing devil's advocate here, she never considered them her real brothers in her first life, just bullies really, or a necessary evil to put up with in order to live well. And you could argue that those years during her first life were her formative years. It shaped how she would view them from then on. Also, Eugene was taken out of the picture fairly early on in this life as well.
I don't agree with what I just wrote, but it is another way of viewing things.
And tbh, ethically there's nothing wrong with two siblings (biological or otherwise) getting together so long as there is no power imbalance, coercion, or reproduction. Socially, in our modern age, it's iffy. Edit: consenting adults obviously, which they both are/will be soon
Personally, I'd like her to go with Johann, but Korean authors seem to really like the fmaily-turned-romantic right now