The final piggy star rating is 50/50, either it's one parent or it's the other. As long as you find a single 2 star, just keep on breeding until you get more, then cull the rest.

Do beware that the world beyond your vision range is frozen. Some items (like smelters) will hyper fast forward when the region reactivates, but animals can not. There aren't any items which force a zone to stay active, sadly.