The Domestic Violence Against Women Act is another federal preemption of the field of family/criminal law that used to deal with domestic disputes. I am not admitted n Washington State but here is my take on the national effects of the federal statute.
I believe the drive for federal dollars by the state will cause your family member to go through the process until he takes a plea or tries the case and gets acquitted.
Watch for concurrent custody or child support cases. What the fabricating parent typically does is use domestic violence as a tactical weapon to establish an early claim for a child. A prosecution for domestic violence also serves as an eviction order against the loser driving him from the home and giving the fabricator the litigation advantage. Under federal law imposed upon the states, the possessory parent gets child support, builds on that status in a custodial proceeding and then walks off with the win.
Because the unfair prejudice available through an allegation of domestic violence, your brother must win his case in order to even be able to come from behind if he has a custodial/support case.
If he does not, he is very lucky.