I'm looking for a manga I've read some time ago, but can't recall neither its title, nor its author. It was a short story (not sure if one shot, tho) told from the perspective of the boy(straight) who was describing gay relationship of his older brother. I also remember there was an ill grandma.
Any suggestions?
Thanks in advance.

I think you might be referring to Ani no Hanashi.

You were a huge help, thanks, that's the right one 😃