Page numbers refer to the hardcover edition.
If they wanted to leave her to a convent, there were many convents in Paris to choose from. So, if they had been in the Game, and knew what the 8 on her hand meant, that would explain why they left her to Montglane instead of a convent in Paris. But it is also possible that they were not in the Game, and that they sent her to Montglane for some other reason. It is also unclear how well they knew each other. If the Abbess had seen Mireille when her parents were still alive, she would have known immediately what the 8 on Mireille's hand meant, so she could have influenced Mireille's parents' decision.