Outdoor wear for twin 6yr olds.

Hi peeps I'm new here so my first post is regarding my 6yr old little twin sisters and getting them dressed appropriately for the outdoor weather conditions.

When they are outside walking to and home from school, our parents want them to have jackets on and hoods up on unpleasant days, only during the sunshine are they allowed no jackets on.

But, the girls will not obey one of our parents demands which is that the jackets have to be zipped up, so how can I get them to obey that rule set by our parents?