
The uke and seme are not brothers. And here's why I think so...
Firstly the seme's dad met the uke's mom when he was still in college STUDYING. If they were to have a child at that point I bet the mom would have to stop school and take care of the uke and the seme knowing how much he loved the mom would definitely be very involved in the uke's live and take responsibility.
Next. The dad already had a wife but he didn't love her thus they wouldn't have sex cuz at that point he was madly in love with the uke's mom. So the seme wouldn't come into play, and if the dad and wife would have a child i wouldn't think he would cheat cuz now he's a dad.
After they separated the dad had no other choice but to produce an heir and the mom went back/into the sex industry where maybe one of his customers probably got him pregnant.
Thus the possibility of the seme and uke being of the same age. Because yes the dad may be a little status hungry about being an alpha but the story did unfold that he was not a douche bag and that he love the uke's mom dearly causing him to cry finding out why the uke's mom actually left him...

So here's what I think... at the end where the seme died, in order for his mission to be cleared and him not disappearing into thin air he did take the seme's soul as promised but in order to save the seme's life, he sacrificed his own soul and give it to the seme. Thus now the seme has the reaper's soul instead therefore I think this is why the seme is able to remember the reaper because it had the reaper's memories and all which is also why he didn't freak out when a random guardian angel appeared.
For the reaper the reason why he cannot remember shit may highly be because he gave his soul to the seme which has his memory and all. And because the reaper did not fail his mission he was not disappeared to think air and also because he is not human he cannot die so I think he was just given a new soul instead. And why he became a guardian angel instead may be because he became pure and knows nothing about being a reaper.
This is just my theory

Hmmm finally everyone got what they were waiting for but honestly I kinda feel bad because the hyung still goes to the other guy for pleasure and he's doing it with the younger seme just to test it out? It feels kinda half hearted and feel like the younger seme's feelings were just being played with... until the uke can really fall in love with the younger seme my heart feels heavy as to why he did it. But hey it could be the start of something right? *just my 2 cents*

Your right

It's nice to see the other guy's story (back story) it could have explained why his lie count was so high, it could be because he was trying to please people with words which made him popular (the professor said you cannot judge someone by the way he lies) and the reason could be because he never gotten attention from his parents,that he craved it.<~ this could be explained by the way he reacted when the tutor decided to quit on him.
I hope you guys understand what I'm trying to say...
He keeps falling in love with family members huh?(โงโโฆ)