I love reading peoples thoughts and saw some here, speculating on what Sung Kyu (the adopted brother) felt especially during the ending scene where his smile disappears. So I decided to give my own two cents. In the beginning it felt like it was already shown to me that Sung Kyu was pretty apathetic and didn't necessarily feel strongly towards acting a specific way to be liked. Barely being able to choke out the words 'it's delicious......... mom'. He wasn't shown trying to connect with Sung woo other than randomly picking him up at school once either.
The first time Sung Kyu shows assertiveness is when the shower scene happens. Because of this detail I didn't feel like he tried to appease Sung woo like someone else did. (which is why I like seeing other peoples perspective since it can be so different!)
I also don't think it was suppose to be about sexuality at all and whether or not Sung Kyu is attracted to men in general. For me this was more an exploration on how Sung Kyu didn't really feel anything and therefore tried to fill the emptiness within him with a new family and the shower scene mostly to me looked like 2 very empty and emotionally tired people that desperately needed to feel 'something', just anything. But just like most one night stands, it didn't fix the initial problem, that Sung Kyu felt a loss of identity and therefore remained apathetic after not being able to find it with his new family either.

Thinking back, Everyone in this movie seemed to fill up the emptiness with something, The mother keeps filling the emptiness the loss of her son caused her, by adopting multiple people. Sung Woo seems to fill up his emptiness coming from a broken home with physical affection from a significant other. And Sung Kyu as a grown man being adopted into a new family with a new identity because he feels like a nobody.