Belgium schengen visa refusal appeal

Hi everyone,

I got my first schengen visa from belgium last year. I reapplied this year. However, I didn't have an HR letter to submit as I'm currently unemployed. So I said that my sister who will be travelling with me will be sponsoring my trip. My application was forwarded to fps. After 64 days, finally they responded to my emails and said that the decision is rejected and they will send the reasons and appeal options in "optimal time".

Would you advise me to appeal? I don't know now the exact reason for refusal but obviously it's because I didn't submit an hr letter or my bank statement. However, I thought my sister's sponsorship would be enough especially that she's travelling with me. Any experiences with appeal and is it for free or a paid service?