Guardianship / power of attorney 16 yr old step daughter

[sf1966]

This issue is taking place in Washington State.  My STBX left our primary residence almost 3 months ago and I became a de facto parent to my 16 year old step daughter that has been in my care since the age of 4.

NOW.... it seems as though my STBX has decided to play "mommy" due to pressures involved in our case although my step daughter is highly adamant that she, in no way shape or form, wants to live with her.   Mom is insisting that she does even though her father still has joint custody concerning her.

My STBX is not willing to let me have guardianship, but I have expressed to the court an impending need in case of emergency.

Can I get her father to consent to allowing me guardianship /  power of attorney without mom having a say in it?