I have applied a Schengen visa to Italy before when I was a student.
You can show the document to prove that you will return home after visit by asking your school/university to give you a Certificate of Study.
This letter will state "For the attention of the Italian Embassy" and contains your full name, date of birth, detail of your course including start and end date of your course and your residential address. It also have your school/university logo on it.
This is the list of documents that you should provide them when you apply for a visa.
- Appointment confirmation
- Visa application
- Copy of passport
- Copy of ID card
- 2 passport sized photos
- Introduction letter (To introduce yourself and tell them why you want to go to Italy)
- Itinerary plan
- Travel insurance
- Flight tickets confirmation (Do not pay for the ticket before you get the visa)
- Bank statement (6 months)
- Invitation letter from your sister
- Proof of relationship with the host (This could be your sister's letter and a copy of her passport)
*** If your parents or your sister pay for your trip, mention about it in the introduction letter and show them your sponsor's bank statement and a copy of his/her passport.
The main thing is that you can show letter from your school/university that you will go to Italy on holiday and you will have to come back to finish your course.
Hope this help..
Find out more detail here : https://www.schengenvisainfo.com/italy-visa/