Is legends of runeterra canon???

Yep, secondary characters not portrayed in League are actually present in Legends. So, people like Yone (Yasuo's brother) and Ledros (Kalista lover in life) get to appear in the card game.

If you mean the actual game being canon, like god poros trapping champs in cards and making a game around them, no, most likely not.

But all the cards are definitely canon, they have so much detail behind them and have a ton of actual lore side characters. Honestly just scrolling through the cards as a lore fan is fun af. Remember: Runeterra is a huge world, characters from legends of runeterra don't have to be pre-existing characters like Yone or Tianna to still be canon.

Yasuo's brother does exist though.

Just because a character is in the card game doesn't mean that they are alive in the lore.