Mental health issues and questions of drug use can certainly be considered, though what is most important is proving if and how it affects her ability to parent. The criteria used for determining a parenting plan can be found here in the statute: http://apps.leg.wa.gov/rcw/default.aspx?cite=26.09.187
And permissible grounds for restrictions in parenting plans here: http://apps.leg.wa.gov/rcw/default.aspx?cite=26.09.191 (most of this section deals with mandatory restrictions for things like domestic violence, but discretionary restrictions are towards the bottom and include certain impairments that adversely affect parenting abilities)
Often if bases for restrictions are alleged, the parties may request, or the court can order on its own motion, that a Guardian Ad Litem be appointed to thoroughly evaluate each parent's household and their ability to provide a healthy environment for the children.
Unfortunately your research is correct about children's preferences for custody--determining what living arrangement is in their best interests is up to the court, and WA courts do not permit children to participate in these proceedings. However, that does not necessarily mean they won't get a voice. If a GAL is appointed to look into the parenting issues, that person will be able to get a feel for the whole picture, including speaking with the children. So although their statements would be inadmissible in court, their opinions will likely be taken into consideration by the GAL, and the court takes GAL recommendations very seriously.
One thing that isn't clear is what type of proceeding you are involved in--since you are remarried, there must have already been a divorce, and with it a parenting plan would already have been established, so I'm guessing your husband is attempting to modify the parenting plan. It is much more difficult to change a parenting plan than to get one put in place, and it generally requires a substantial change in circumstances or threat to the children's well-being that was not present or known at the time of the original order. (So if these mental health issues were before the court originally, the court will not now use them to restrict her parenting time.) Rules for modification can be found here: http://apps.leg.wa.gov/rcw/default.aspx?cite=26.09.260
These are all complex issues, so you should seriously consider hiring counsel.
Kate is absolutely correct -- you reference stepdaughters -- you cannot obtain custody of children that are not yours biologically unless there is a third party custody or an adoption roder already in place. You need to schedule an appointment with a family law attorney to work out these issues because it's not making a lot of sense the way you have framed your question.
The information is for general information purposes only. Nothing stated above should be taken as legal advice for any individual case or situation.
Mental health issues are relevant, but the impact of those issues on the children is the aspect that really matters. Marijuana use even though it is now legal in this state is treated in the same manner as alcohol use, which is also illegal. If marijuana use interferes with a party's ability to perform parenting functions, then the court's determination will more likely be in your favor. Parenting functions is defined under RCW 26.09.004(2). Criteria for temporary and permanent parenting plan are found in RCW 26.09.187 and 26.09.197, respectively. You are correct that the other party's profession is a fact in her favor and that it will make what you/husband seeks more challenging. The way you want to present your case is as follows (1) the existence of the other party's mental health issues and marijuana use is fact (for it to be considered fact and not just a suggestion you will need to include as much properly procured evidence of this) and (2) the impact on the children has been negative (include all instances, examples, facts, circumstances that have occurred that bear out that conclusions). Now, the above being said, you do have to be careful about how you are discussing the issues with the children. There are many parenting plans that specifically state that no parent is to discuss court issues with the children or attempt to influence their decision about where the children want to reside. You will want to check the language of the current parenting plan to make sure you have not violated this provision. The only real way to get the children's opinions in court is to appoint a Guardian Ad Litem, who is a neutral party that represents what is in the best interests of the children. Because they do not represent either parent, the investigation by the GAL will be an investigation into both home environments and to the allegations made. The court of a Guardian Ad Litem (initial deposit) can be anywhere from $2,500.00 - $4,500.00) and may be more if the costs of the investigation warrant more efforts/costs. The type of action your husband/you would be undertaking is a modification of the parenting plan. Modifications, in general, are the most challenging to prevail with. The reason is that the statute indicates that orders shall not be modified, unless . . .. In essence, there is a presumption against modification unless established otherwise. The modification statute provision worth consulting is under RCW 26.09.260. You should consult with an attorney regarding the facts and corroborating data that you have so he or she can get a full picture and advise you accordingly to the best of his or her abilities.
Karen C. Skantze practices in the State of Washington. The response is limited to her understanding of law in the jurisdiction in which she practices and not to any other jurisdiction. No response to any posted inquiry shall constitute legal advice, nor the existence of an attorney/client relationship.