You need to listen checkboxes change event too. Try this code:
var $input = $('input'),
$check = $input.filter(':checkbox'),
$register = $('#signin');
$register.attr('disabled', true);
$input.on('keyup change', function() {
var trigger = false;
$input.each(function() {
if (this.type != 'checkbox' && !$(this).val()) {
trigger = true;
}
});
$register.prop('disabled', trigger || !$check.filter(':checked').length);
});