Like everyone else, I started off thinking that this was going to be about two fully-blood related siblings, but now I'm pretty sure they're not. They start off like every step-sibling "incest (not really)" manga, so this isn't really forbidden love, just love that's not as widely approved of.
Actually, maybe it's not that Yuzuru or Ai is adopted, maybe they could both be kids from a previous marriage. In any case, still not blood-related.
Spoilers on why below:
After Yuzuru said he would date Ai's friend (the most insulting thing he could have done to Ai vs. going for a complete stranger), I could pretty much predict how this is going to go. Yuzuru's friend will realize that Ai likes Yuzuru, be all: "Why don't you date me to get rid of your sadness?" And Ai will date him, but still want to be with Yuzuru. And meanwhile, Yuzuru will feel NTR rage (which he wouldn't have without Ai dating someone else), and be all like: "Why are you dating that guy?" He and his friend will have a divide in their relationship, and meanwhile, Ai's friend may/may not suspect something's going on/weird vibes between Yuzuru and Ai. Then one night/rainy day, when Yuzuru can't take it anymore seeing Ai with his friend, he'll corner her and kiss her again, and make some cryptic remark about loving her which is really obvious to everyone but the naive Ai. Then she'll wonder about it, and then after a while, she'll apologize to his friend and break up with him. But then Yuzuru will have to get rid of Ai's friend. And her friend, by this time probably, will have found out Yuzuru's feelings for Ai and be devastated. She will then blackmail Yuzuru/bully Ai, and Ai won't understand what's going on, she broke up with her boyfriend, why hasn't he split with her friend, blah, blah. Then the next drama hurdle will be their parents finding out about their relationship and being completely against it, saying they never should have moved back in the first place. Then Yuzuru will find out he's adopted (or Ai will) and be all 'SHOKKU' and all weepy mode. And then Ai/Yuzuru (depending on who ISN'T adopted) will go comfort the one that is and be all like: "You'll always be my family, and to prove it, I'll marry you." Then cut to the scene of them getting married. THE END.
The only interesting thing left about this manga is seeing when each of my predictions will come true. It'll probably take awhile because the mangaka spends her 20 pages mostly wasted on expressions and not enough plot. Otherwise, read any other step-sibling love manga to find out what will happen (I suggest Eden no Hana).
Since this will be a by-the-book execution with stereotypical characters (innocent little sister and the perfect, prince-like big brother) I no longer care what happens to these characters.