Invitation letter for tourist visa application

I want to invite my sister in-law here in china,and she's only 18yrs. old to provide the bank cert. or statement of account. She just graduated in high school and don't yet have her own account, and if her parents will be asking to provide the said documents also impossible because they also dependent on us.
Now my question is How we can get this invitation letter? can you send me the list of travel agencies here in Guangzhou, or Dongguan city China that authorize to provide this letter. TKS