I don't know much about the JEE exams, but the high school diploma from most countries is not eligible for admission in most German universities. You normally require your high school diploma plus two years of study at a university to be eligible. More importantly, you should have pursued some mandatory subjects in your high school diploma. Most universities also require you to write IELTS. This is of course if you want to pursue an undergraduate course in English in Germany.
Yes, you can opt for a sponsorship letter if your sister is a German citizen and has a well-paying job to support herself and you at the same time. In this case, you don't need to show a blocked account, but your sister must be in very good financial standing before she can be eligible to give you a sponsorship letter.