Probate is often a more complicated process than laypeople realize. You should really have an attorney. In this case, where there seems to be some conflict among the heirs, maybe each of you should have an attorney.
If your mother is that afraid of your sister, a power of attorney is the least of her concerns. Your mother should probably report your sister to the police and Adult Protective Services. Furthermore, if your mother still has mental capacity, she should hire an attorney and sue to have your sister ejected from her home.
If your sister is, indeed, dangerous, no power of attorney ever drafted can keep you safe. By being there so long, your sister likely has an argument that, because nobody...
Special Needs Trusts are very tricky instruments, even for most attorneys. A mistake in drafting or funding a special needs trust can be very costly. Please do not try to do this on your own, but rather retain the services of an elder law attorney who knows these kinds of trusts.
You can search for a case in Washington on the Washington Courts website http://dw.courts.wa.gov/
Click search by name. Then there will be drop-down boxes for the county in which she lived, the year, the court (you should select "Probate/Guardianship") and her first and last names. This should allow you to do exactly what you are looking for.
Hope that helps.
You can contact the personal representative and request notice of most things. Also, nonintervention powers does not mean that no court approval is required to complete the probate. That is still required. If you are really concerned about it, you can hire an attorney to represent your interests in the probate.
If the account was owned by you and your mother jointly with right of survivorship and she passed away, the money is yours provided, of course, that there were no other joint owners. It sounds as though you have questions about the source of the money, but without any further details (which you should probably not provide on a public forum) it is impossible to say for certain there will not be any other issues cropping up.
A probate is necessary to transfer title to real property and pay off any debts your father and his wife had. It may be that the debts outweigh the value of their estate. If that is the case, the debts would not be cancelled, but the estate would be insolvent and disposed of as an insolvent estate.
It would be in your best interest to hire an attorney to help you with the probate, especially since you are so far away. You could probably find an attorney in Republic or even Spokane that could...
One thing to keep in mind is that no inheritance exists while your parents are still alive. They are free to do with their money as they please.
That said, if you suspect that your sister is exploiting them, rather than just not talking to you, you might consider contacting Adult Protective Services. If your parents do not have the mental capacity to take care of themselves any longer, a guardianship might be necessary.