The law is going to vary wildly from State to State, so someone from WA is going to have to give you the scoop.
Some things to consider that might affect the outcome:
Did the renters have permission from the family to leave some of their goods behind?
If they did not, was the family aware that they had left goods behind?
If yes, did anyone ask them to remove their stuff?
This and many other questions will determine the level of responsibility you had for their goods and the level of care you needed to exercise. If they left the goods there with the families knowledge or permission (especially if they paid for the privilege), then there is likely some level of responsibility. However, their knowing that the garage did not lock would mean that they assumed some level of risk.
Does WA law allow for partial liability? If it does, I'd guess that some portion of the loss would be assigned to your family. If not, it'll hang on who was more responsible, which takes us back to the questions.
From a moral standpoint: They knew the risks and chose to take them anyway. Holding you responsible ignores the stupidity of their actions.