I'm getting really frustrated, I can't find the name of this manga, and when I google basic plotlines all I get is hentai results (x.x rip) even though its a totally clean shoujo manga.

The basic plot goes along the lines of this; girl dresses in her friends old high school uniform and goes out into the city wearing it, she bumps into her step-brother (not blood-related at all) and he doesn't recognise her, and she goes along with it because she'd embarrassed if he saw her in a school uniform. Her brother is interested in her and gives her his number. She goes back to her friend and explains the situation, and he friend ask her why not use this as a way to get revenge on her player brother? make him fall so hard for this high school girl (i just remembered her name is Mina while typing this 😮 ) and then later break his heart. and yeah the plot goes on. can anyone help?

Have not read it myself but Liar x Liar fits your request. Hope it's what you were looking for.

The person above is correct, this is without a doubt Liar x Liar.