Legal Question in Family Law in Washington

It's not going to be that simple. You are going to have to bring a third party custody petition in Superior Court. To succeed there, you are going to have to prove that both parents are unfit to parent your little brother for whatever reasons exist that make them unfit. Next, you'd have to demonstrate that you are fit and capable to do the job.

If you succeed in proving that they are unfit but do not succeed in proving that you are fit, your plan may backfire as your little brother would then become a ward of the state. I suggest that if you have the means, you consult an attorney about the details of your case before blindly proceeding. Your mother may be bad, but foster care can be pretty bad too.