At the end of the day you might just have to respect your mother's decision.
But if you are determined there are a few things to keep in mind while you explain the pros AND cons to your mum:
* their dietry needs - pellets, vegetables, fruits, seeds just about everyday
* they need at least yearly checkups with an avian vet - general health check, blood tests to determine a base line for health
* training and exercise, they're very intelligent
* they can destroy a lot of things - jewellery, furniture, paper, power cords, anything and everything within their reach
* they are very messy, they throw their food around
* they can be very loud, they do scream from time to time just for fun
* they are prone to being nippy, in my experience they learn how to hurt you if you annoy them eg Misha and Guapo both know the best way to bite a person is to pinch, twist then pull your skin, they haven't done this to me in a long time but believe me it hurts!
* they live for a long time 25-30yrs, I'm pressuming you're rather young, do your future plans accommodate for a parrot? holidays, college, love etc.
I'm not trying to be negative, mean or anything of the sort, I just believe in being fully aware of what this kind of decision entails, I know how easy it is to be swept up in the excitement! You also need to think of how your parrot will affect you family while you're living with them, are they prepared to put up with this list of downsides?
I hope this helps you to gain some perspective, and good luck to you =)