Change form button text

Try this:

function modify_tml_login_button() {
    if ( $button = tml_get_form_field( 'login', 'submit' ) ) {
        $button->set_value( 'Sign In' );
    }
}
add_action( 'init', 'modify_tml_login_button' );

Perfect, thanks Jeff

I had the exact same problem, and this code snippet totally fixed it. Thank you so much!!!