It's not open ended at all, they remain together, they just tell themselves that because they are young and honestly, stupid. But knowing how life goes, anything could happen. But I am more inclined to believe they stay together forever because they both understand eachother and endured a lot but the main reason they may break up is because they don't feel like they deserve eachother's love, as they say at the end, he wants someone more mature for her, and she wants someone more cheerful and without traumas for him, if they can overcome that I doubt they will separate. Calling this manga "mature" can't be more wrong, the sister's whole attitude and cirscuntances, Aikawa who doesn't understand he loves her no matter what, Aikawa's mother ignoring her daughter's cries for help. The subjects and sufferings are mature, yeah, but the way they react to it just screams inexperience in knowing how to cope and act when such things happen, accompanied by lots of bluntness (like the sis saying "you are lucky" when the girl was rworded by her stepdad), which I understand because they are still very young. I loved the manga btw