In FE Blazing Blade, Eliwood can get married to Ninian, Fiora or Lyn.
But the story strongly supports EliwoodXNinian and makes the most sense storywise. And it's the OTP of 90% of the people that played that game.
But it was never confirmed canon. So EliwoodXNinian is just the one of the few fandom's favorite pairing and everybody treats it as if it was canon despite the fact it's still up to each player to decide who is Roy's mother.