class Home extends CI_Controller {
    public function index() {
        if ($this->session->userdata('is_logged_in')==FALSE) {
            redirect('Signin/signin');
        } else {
            $this->customers();
        }
    }
    public function customers(){
        // More code here
    }
}

class Home extends CI_Controller {
    public function index() {
        if ($this->session->userdata('is_logged_in')==FALSE) {
            redirect('Signin/signin');
        } else {
            $this->customers();
            $this->create();
            $this->save();
            $this->edit($customer_id);
            $this->update($customer_id);
            $this->delete($customer_id);
        }
    }
    public function customers(){
        // More code here
    }
    public function edit($customer_id){
        // More code here
    }
    /* The rest of
    the functions
    here */
}

class Home extends CI_Controller {
public function __construct(){
     parent::__construct();

     //instead of checking the user whether he is logged in, in every function, check it in the constructor
     if (!$this->session->userdata('is_logged_in')) {
          redirect('Signin/signin');
     }
}

//function to show customer lists
public function index() {
    $this->load->view('customer_list_view');
}

Now if the user wants to add a customer, add a function to show the form
//customer add form view
public function add_customer(){
    //show form view
    $this->load->view('customer_list_view');
}

//save customer
public function save_customer(){
   //save or update based the availability of the user to the database
}

//delete customer
public function delete_customer(){
    //delete the customer here
}