In the film why did Miranda end up giving the recommendation for Andy despite the latter quitting on her in Paris? It's actually because Miranda realizes that Andy's greatest strength is her loyalty. Despite being in a job she hates, Andy comes to admire Miranda and see the method to her madness, even comforting her in a Moment of Weakness. And what did Miranda do in the car after screwing Nigel over? Tell Andy that while she is touched, loyalty doesn't mean much in the fashion world and that Andy's already learned to screw people over, say with Emily. Miranda's not stupid, so she deduces off-screen that when Andy leaves without a word that her words made Andy realize there's no point to being loyal to Miranda and left the latter without an assistant for Paris. Oops. And Miranda is right, but she knows that another boss in another industry will be lucky to have that loyalty, hence why she says only an idiot wouldn't hire Andy.
In The Devil Wears Prada, Miranda is a woman in her mid fifties (maybe even 60s) who somehow has 10 year old twins. The reason why would be because her obsessive focus on work carried over into her personal life, which is why she can't stay married and why she had children so late in life. It's not unlikely that Miranda would have undergone fertility treatment due to both her age and workaholic tendencies to get pregnant. Women who utilize fertility treatment to get pregnant frequently end up with more than one child.