Who do you contact to have your elderly parent tested for mental competence?

It may not be a result of incompetence as much as it's a dysfunctional/codependent relationship between your mom and your sister. Show me an addict and I'll show you at least one person who is enabling the addict. Enabling your sister doesn't make your mom incompetent. And if your mom is competent she's allowed to make bad decisions.

If you do a little digging you'll find numerous questions here regarding how to protect an elderly person from an adult child who is an addict/alcoholic.

You state: I am the power of attorney if my mom isn't capable.

If your a POA and your mom is found mentally incompetent the POA is dead POAs require the person's approval to be used. I'd suggest talking with an elder affairs lawyer to clarify your legal powers for your mom.

What a POA is … and isn’ r

It sounds like there is more than one issue here. First, what do you want to accomplish by having mom declared incompetent? Are you seeking guardianship ? Do you want to put her in a facility, or move her to your home and she doesn't want to go? I'd talk to her doctor about how to get the ball rolling on that process; it varies from state to state I believe.

If your sister is stealing from your mom, you should report this to the local police. You could also call APS and report that your mom is a vulnerable adult who is being victimized. They may have some better suggestions.