If I already retain POA, do I need to apply for guardianship?

Guardianship is awarded by a court order, and can only be modified by a Judge. POA's can be revoked and rewritten by the person who granted them. There are often POA battles among children that end up in court and it can get really ugly and expensive. Ultimately the Judge appoints an independent third party that NOBODY likes.

I have durable power of attorney for my mom my sister lives in Arizona and mom stays with her occasionally. I live in Florida and my sister wants power of attorney in Arizona what does that mean for me.

I'd get legal advice, since the laws of states vary, but I would consider if the sibling has a case. He will need to prove incompetency. Is your POA Durable?

The court could appoint a guardian, but not chose the one who filed. The person who is appointed could be another family member who is willing and qualified or someone approved by the courts for such things. There are requirements to be appointed. Some require state residency and bonding.

Do you intend to intervene and ask for consideration? The court would likely want your input if you have been acting as the POA.

Yes, a sibling can still apply for guardianship even if you hold POA. A guardian overrides the POA. Do you know why the sibling wants to be guardian even though POAs are already in place?