From what we've seen in Midnight Sun (when Edward watches Bella sleep for the first time) vampires falling in love
is instantaneous, totally consuming and absolutely permanent. Though it can only happen if the vampire lets down all guard completely, which goes against every instinct, especially for a Newborn. Edward tried to fight it with sheer will-power and made it for almost a month.
And Bree had no choice, but to put down her guard in that tiny cave. And then it crushed her, when she saw
Diego under the sun. And just like with Carlisle+Esme, it wouldn't have mattered if she knew him for a day or a million years. (as Edward puts it) She would have loved Diego exactly the same way.
Only their friendship, dependency and understanding of each other would have intensified over time, but that is nothing compared to their love.
And that's why Irina couldn't think straight when it came to Laurent, even after knowing him only a year.
She probably blamed Bella for stumbling around in the woods with her scent, while her love was suffering
from his thirst. But she was somehow willing to bite down her bile, to go apologize to Carlisle, only to
find Bella with a werewolve and an immortal child playing in the woods, worse than her worst nightmare.
Of course she had no choice, but to turn Bella in.
She was trying to protect her sisters by keeping them out of this, just like her mother did so long ago.
And maybe she was hoping to save the Cullens too, if they were smart enough to abandon that monster Bella.
She was totally ignorant, just like Rosalie in the beginning, not able to accept that Edward truly loved Bella.
She had sex with humans for centuries, never imagining to love anyone, like she loved Laurent.
Bella must have bewitched Edward and the Cullens with her scent and gift somehow,
and surely Aro, Carlisles old friend, would somehow find a way to save them from her.
A totally messed up situation.
Damn you ,Laurent , to the deepest pit of hell where you belong.