How can I press charges against my sister's boyfriend for domestic abuse using texts she sent me giving detailed information on how he's hit her?

If you have texts from her saying he hit her, you can take them to the police department where the crime would have occurred and see if they will go talk to her.  Her texts stating that he had done something really isn't enough evidence to get a warrant.  She will need to tell them more specifics about what happened.  Now if you had texts from him saying he had done it, that might be a different story.  There are women's shelters that she could go to, that will give her protection and will help her get him arrested.