visa requirements and charges to come to China

I am studying in China.My university invites my family on behalf of me to stay with me for 1-2 months.I have sent that invitation letter to them. What should they bring to the Chinese Embassy to take visa? Does it require an english translation of our marriage certificate? Please inform me how many days it takes to process it and about charges (for my husband and two sons). Do they need to submit bank statement? I am bearing their whole expenses in China