you can go and try to find clues where the brother specifically says "I want to support their love" or something...
Its literally the title lmao.
Which is entirely non-indicative of when he becomes aware of it in-universe. Or even if - titles even when framed as first-person statements of a character have no inherent obligation to literally reflect the narrative as opposed to being more thematically and/or symbolically descriptive.
Which said he seems to have more or less figured it out by the end of this chapter and asks Saki as much, though it's still anyone's guess how far his grasp of the idea of "liking someone" goes. I reckon he'll be digesting that one for some time.
¯\_(ツ)_/¯