I don't practice in Okanagan but after 14 hours sinceposting your question I suspect you are anxious for a response. The theory is that victims/alleged victims often recant sometimes under pressure from the perpetrator. Consequently, the fact a victim wants to drop charges is normally not a basis to drop charges
Recommend you contact http://www.dshs.wa.gov/pdf/Publications/22-787.pdf and discussyour options--even if the accused is not a relative as well. They can help you better see how misguided your relative are--casting the accused as the victim instead of you.
In many cases (I am NOT a WA state attorney) the procecutor makes the 'go-no go' decision based on the strentgh of evidence and sometimes victims are the subject of a material witness warrant to ensure they testify to the truth of what happened.
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A letter unlikely to convince the prosecutor to drop such serious charges. In addition, your relatives may be committing a felony themselves ("tampering wIth a witness"). You need to get out of that situation to a safer place. The prosecutor's office should have a victim's advocate that can help you find resources and communicate to the prosecutor what's happening.
It is indeed out of your hands. If there was enough evidence to support the filing of charges, the state will likely go forward even if you dont want to. The state may listen to what you have to say regarding a potential plea offer or sentence if there is a conviction.
The prosecutor's office will probably have a victims advocate and you should speak with that person. You obviously need some support and you are not receiving any from the people who should be supporting you. The allegations are serious and no one should be diminishing what appears to have happened to you.