How to make comment reply available only for login members of a certain user roles only?

You can try below code to achieve your requirements.

<?php
/*check user is logged in or not*/
if ( is_user_logged_in() )
{
    if( current_user_can('author') || current_user_can('administrator') )
    {
        /*your code for logged in user having role author or administrator*/
    }
}
else
{
    /*your code for logged out user*/
}
?>

or you can also have another option.

<?php  
/*check user is logged in or not*/
if ( is_user_logged_in() )
{
    $user = wp_get_current_user();
    $allowed_roles = array('administrator', 'author');
    if( array_intersect($allowed_roles, $user->roles ) )
    {
       /*your code for logged in user having role author or administrator*/
    }
}
else
{
    /*your code for logged out user */
}
?>