Dads were boys once and they understand how boys think. Dads want people to think well of their daughters to not think they are trash or ******. Dads tend to be conservative and want their daughters to have the best chance in the world so they have preferences in how they dress, act and speak.
He wants you to look better, or possibly less "slaggy." In reality it's probably best to leave you to it. Over-critical and confrontational fathers are the main cause of low confidence in kids, especially girls. Mothers can be a pain in the *** too though.
How do your sisters dress differently than you do? Perhaps your dad needs to understand why you make the choices you do. As long as you're not dressing in a way that he feels is inappropriate (goth or too tight/too short), maybe he just needs to hear from you why it's important to express your individuality. Just a guess. I have a daughter who sometimes makes what I feel are "odd" choices but we talk about it. Sometimes she wins, sometimes not, but it's always worth the conversation. I hope you're able to talk it out.
There's a high probability that you WILL be influenced by your sister's, love. Especially at a young impressionable age. He wants you to make better choices than them, obviously.
Because he cares about you and what happens to you.
Good for you !