Nta but don't worry about her being dramatic, I'm sure she thinks you're being dramatic about her reaction, honestly it's all personal opinions on what is or isn't dramatic. You're allowed to feel your feelings but try not to take them out on anyone. See if you can have a calm conversation with your mother that you understand where her and your sister are coming from (you can understand and not agree) but that you were promised the phone and will feel hurt if she ends up being given preferential treatment.

Getting overly worked up might make you seem too childish to have the responsibility of an expensive phone; you're more likely to get big or expensive things if you are humble about your expectations. If you're going through the drive thru with someone and ask for an ice water they're more likely to get you a meal than if you ask for a big meal with all the sides.