Related to a mindset. Like a sister!

You haven't mentioned what you are studying. Lack of interest towards studies can have many reasons, the stream may not be of your choice, you could have lost basics in some subject; hence finding it difficult to cope up that makes you disinterest, lack of concentration in the classroom due to other factors (personal issues) etc. First and foremost, you need to identify the causes and work on those systematically, provided you are serious to accomplish your academics. Create a strong "Goal Statement" and set priorities linked to the Goal Statement. Work on those priorities. When you are leading life without purpose or goals, your mindset will be really not good; it will be hopping like a monkey hither & thither. Thinking about girls as a sister is not really a serious issue. Continue to be just a friend or sister. You have other challenges of Self-Development which are burning issues now. Consult a Counsellor in your city for professional help.