Not meeting the residency requirements with a valid PR card is a grey area. When you arrive in Canada and they question if you meet the requirements and probably have some data on your travels, they could report you for not meeting the requirements which means you have to appeal to keep your PR. However, they often seem to let people in without reporting them. If that happens, it is perfectly legal to them to live in Canada even after their PR cards expire and not renew until they have 2 years and meet the residency requirements again.
However, this is also not something you can trust to remain the same for all eternity. They could always toughen the rules if they feel like it. However, you always have the back up option that as long as you meet the requirements, you can sponsor your family again.