How to convince my parents to get a puppy?!PLZ HELP, PARENTS!!!?

I have been wanting a puppy ever since i understood what a dog was!! My favorite animal is a dog, my favorite pet is a dog, my favorite fuzzy companion is.........A DOG!!! I just think their sooo cute, soooo lovable, sooooo relatable, and soooo worth the extra time to care for them!! I feel their is a dog out their for everyone!! They come in all sizes, all colors, and all personalities!! Okay, enough about what you already know...... on to my life: I am 12 and my parents have never let me get a dog because they think i wasnt responsible enough. i have tried everything: begging, showing i was responsible, and even made a stinking slide show about my dedication!! But none of it worked, my mom is a softy, but can be a rock, well....an egg (if that makes sense,lol) but, after all, she is bendable (although im the only one who can break her hard surface, only with very carefully chosen words, though) its my step father, whos the BOULDER in the family:(:(:( He had a dog (that i never met) before, but gave her away because he didnt have dedicated time toward her with his work and all. Ever since, he was a marshmallow (idk why im using food to describe my parents?!) around the topic, which i think i could use to my advantage...... another surface i have to break is, my sister. Iove her to death, and i dont want to do something that makes her unhappy:(shes 3, btw) Everytime she gets near a dog that bigger than her, she freaks out, runs to my mom, and hides behind her leg. Shes fine if shes far and says "look, a doggie", but is still a little jumpy. Everytime i see a dog i try to get her comfortable around them, and point it out to my mom, but fail when she hides again:( Another thing is that i have a pet guinea pig, Sundae, and if i do endup getting a dog (inshallah