The specific wording of the Washington statute is: "(1) No person in the state shall be placed in legal jeopardy of any kind whatsoever for protecting by any reasonable means necessary, himself or herself, his or her family, or his or her real or personal property, or for coming to the aid of another who is in imminent danger of or the victim of assault, robbery, kidnapping, arson, burglary, rape, murder, or any other violent crime as defined in RCW 9.94A.030."
The statute provides no non-judicial means for ascertaining whether a person was "protectiing by any reasonable means necessary . . .". So if you have not been tried in the criminal courts and acquitted on grounds of self-defense the relatives can still sue you and you still have to pay for a legal defense. Further, unlike a criminal trial there is no specific provision that you are due compensation for a civil trial in which it is ascertained that you acted in self-defense. So to actually obtain protection under the statute you have to persuade the district attorney to bring criminal charges and go to court so you can get acquitted by reason of self defense. It's going to be an uphill battle to persuade him to do that--generally DAs don't like to waste the state's money on cases that they know they are going to lose.
If the statute provided, for example, that an affadavit from the district attorney stating that he did not bring charges because he was certain that you would be acquitted on grounds of self-defense would suffice as evidence that you did so then things might be different, but there appears to be no such mechanism provided in the statute.