It is excessive to add opening and closing php tags on every line of code and they are not needed.
<?php
if (have_rows('parents')) {
while (have_rows('parents')) {
the_row();
if (get_sub_field('father')) {
the_sub_field('father');
}
if (get_sub_field('father') && get_sub_field('mother')) {
echo ', ';
}
if (get_sub_field('mother')) {
the_sub_field('mother');
}
}
}
?>