Why has my mum bin lying to me?

Okay so today i was in her room asking for this pc i wanted (you'll see what pc i want in my other questions) and she just laughed and to me thats like "Oh i dont have enough for that" so im like "We'll see about that" i didnt say that to her but i was tempted to, But guess what i found when i went through her handbag....DUN DUN DAA a **** load of �10!!! :S i know shes been saving up for a car a long time and stuff and she doesnt want me to go out and work because im apparntly to young and whatever thats probably a lie i suppose but this is BS, she cant lie to her own kids!! its ******* wrong!! im mad now and upset and i dont want to believe her anymore, and believe me this isnt the first time she lied to me about not having enough money for something i want, My sister and brother get what they want but im always last which pisses me off because im the most loyal in my family and my brother is fat and does nothing and he gets paid for it, My sister throws a fit and swears at my mom and she still pays her! and my sister has work that pays her so correct me if im wrong SHES GETTING MORE THEN ENOUGH!! god damnit it just pisses me off because i feel like im left out since i have no interests like my family has, And i need help I dont know if my mom has just bin lying to me or anyone else but it only seems she picks on me and i dont feel to well about her lying and really i dont know if i want the PC anymore because my dad wont pay for it My mom DEFO wont pay for it since she keeos lying and well i got no body else to turn to I need help please :(