Hey hey hey, I don't believe that she, someone mentally 20 years older than Euguene, would smash him. Not to mention that he is already married and polygamy probably isn't accepted in this universe.
They were total assholes for like three decades. I just don't think that their sibling affection would go that far. She was married to Johan so she will probably outright decline any advancements toward her. Also, an overprotective family wouldn't make a move on her, it's just so highly unlikely that even though it may look like it right now, it most likely won't happen.
They are still underaged too so don't start with yall's wild fantasies. -_-