My Sister has put a petition for appointment of a guardian and estate of our mother. How will this affect me as only sibling?

If you truly want to direct your mother's care and financial affairs, you would have to file a cross petition to be appointed her guardian and substantiate why you, rather than your sister would be more appropriate to be the guardian. If you cannot afford an attorney, the forms to file this petition (and other necessary paperwork)are on the web if you search for Washington State Courts forms.

The only way for you to be involved in everything that happens with your mother's healthcare decisions and financial decisions is to file your own petition to be named as the guardian of the persona and estate. While you can find these forms online and therefore start the process, the largest single determining factor of who will be named as Guardian is the Guardian Ad Litem's report to the court. The Guardian Ad Litem is charged with making a determination of who is best equiped and best suited to serve as Guardian. If you are not in King County, or Washington, you will have an uphill battle to show that you can effectively serve from a distance.

Unless you can show a good reason why you should be named, instead of your sister, you will likely not be able to prevail.

I recommend that you seek a competent elder law attorney who can hear your specific facts and circumstances and advise you properly on your rights.