I've forgotten where I read this, but from what I remembered her mother Therese promised another daughter to be married to Louis and the daughter died. So that was one of the reasons why Marie Antoinette was unprepared to be queen, her education was neglected.
Is this true?
Sort of true.
What happened is that 2 of Maria Theresa's other daughters died before they could be married to their intended grooms, leaving her to shuffle the rest (including Antoinette) around. Maria Johanna was initially going to marry Ferdinand III of Naples, but Johanna died of smallpox in 1762 after an inoculation before the marriage took place. Then, Maria Josepha was engaged to marry Ferdinand of Naples--she too died of smallpox before the marriage took place. Maria Theresa gave Charles III of Spain (Ferdinand's father) the choice between the 15-year old Maria Carolina and the 22-year old Maria Amalia, and Ferdinand chose Maria Carolina, who was only a few months younger than himself. Maria Amalia, being closer in age, was married to the duke of Parma; Joseph II also supported this marriage, since his deceased first wife was Ferdinand's sister. The only daughter left for France was Antoinette.
So, none of the sisters were originally engaged to Louis-Auguste but if the other two sisters hadn't died, it's likely that a different daughter would have been engaged to the dauphin of France.