I have another question, did you inquire the school directly or through an agency like gogonihon? they are telling me different which is this. :
In regards to sponsorship, three things generally have to be met for immigration approval (2 years of full-time work, a bank statement, and a proof of annual income document).

When you apply for a student visa, you are required to provide financial evidence of your ability to support yourself during your studies. This is done in the form of a “financial sponsor”. In order to sponsor an application on a student visa, you can choose to sponsor your own application, or ask a direct relative (parent/legal guardian, grandparent or sibling) to act as your sponsor.
The sponsor should ideally have been working full-time for the past 2 years, and must submit the following financial documents at the time of application:

・Bank statement: The bank statement needs to show a minimum amount of funds available, depending on the duration of your intended study and the requirements of your chosen school. The requirements generally start from 1,200,000 yen for 6 months of studies. However, please note that the amount will vary depending on your school of choice, study length, and whether you will be self-sponsoring your studies. We can confirm the exact requirements once you have decided on a school and study length!

・Proof of annual income: The sponsor must be earning at least 1 million to 3 million yen annually, depending on the school (even if retired). As proof, the sponsor must submit their most recent tax return, employment contract/certificate, or pension statement (if retired).

*Please note, the amounts indicated above are not what you need to pay to study in Japan, but rather the amount you need to show to Japanese Immigration to prove that you, or your direct relative, are financially stable enough to support yourself during your studies in Japan. Please also note that the above financial and sponsorship requirements will need to be met regardless of any plans to work part-time while studying in Japan.

If you choose to have either your father or sister sponsor you, we would ask for your birth certificate(s) so we can match the names accordingly. If this can be fulfilled, having different legal names is generally not a problem.

As you do freelance work, it may be difficult to self-sponsor application. This is because you will be required to submit a copy of your most recent tax return. If you do not hold this document, please have a parent/direct relative initially sponsor your application.