What should be done for a valid marriage contract if the father of the fiancée does not pray?
Praise be to Allaah.
So long as her father does not pray the five daily prayers, he is not fit to be her wali (guardian) in contracting her marriage. Look for someone else who could be her guardian, and if it is her brother mentioned in the question, then he can contract the marriage on her behalf. And Allaah is the greatest help.