It's hard to support a family, and these days it's even more difficult for many families. It sounds like your mother is STRESSED about bringing in money, and it probably makes her panic when she sees money going out so quickly... and your asking for things probably triggers that panic and frustration.
How about this: Give her a hug and tell her you appreciate everything she does, provides, and sacrifices for you. Tell her you miss her, and you suggest going shopping because you want to spend more time with her... then suggest that you maybe go for a walk or watch a movie on tv together. Propose something free.
Even if it seems like your brother gets everything he wants, be the big girl in the situation. Be the one who doesn't demand so much of your mother, and she'll appreciate you more for that. You're 13. You're old enough to take on more responsibility around the house. If you really want to help improve your relationship with your mother, put on some big girl pants, demand less of her, and help her more. She'll appreciate your maturity, trust you more, and that can only do good things for how you two interact.
... and do what Rob says.