Time period to apply H4 VISA


My sister is getting married in February 2018, My brother-in-law is a H1 B Visa holder working in USA. He is travelling to US in a month after marriage, So I would like to know when should we start the process to apply H4 for her, Do we need to start some background file processing before her marriage ? She has only one month of time after her marriage, is it enough for her to get H4 . Please help.