I have POA Durable, but my sister has filed for guardianship, and is refusing to let me pick up my mom from her house.

During the Guardianship hearing process, mom has a Guardian Ad Litem. It is up to the GAL whether you pick her up or not.
You also get to have your say during the show cause hearing.

What reason does she give? What state are you in? What does mom want? I'm assuming she has dementia or all sister would have to do is have your DPOA revoked and sign a new one. Filing for and having guardianship are two different things. All the posts on this site say that filing for guardianship and getting it is very expensive and takes a long time. Perhaps you should call the attorney who prepared your DPOA and ask for advice. I have read horror stories about families who get in battles over a loved one and the government ends up stepping in and assigning an independent guardian who runs through the money and blocks the family from seeing their mom. I'm sure the stories have some truth to them. Hopefully your attorney can tell you what next steps to take. Perhaps someone will see this who has actual experience and give you an answer.