Rejection of my sister’s visiting VISA

Hi everyone,

I am from Türkiye living in Germany since 2 and a half year. I wanted to make my sister to visit me in Germany make her to see Germany.

I sent letter from myself stating that I will be responsible from her expenses and stay and returning back. I basically earn 101k per year and sent my past 3 months of my paycheck my apartment adress and etc.

Today we learned that her VISA is rejected. And basically the objection for the VISA rejection is not available.

Do you have any idea what can I do? What can I do to guarantee the VISA will be approved?