For me, the biggest mystery of this manga is not how Uiha/Uraha happened but the story of big bro. Each time we get a part of it it's either depressing, sad or "depressing/sad that turns into wholesome" when the parents/big sis appear.
It's really great that he grew up to be such a good boi, but you can see that he's still broken by his childhood and tries too much to be "normal"(like with his dozens of girlfriends).