My brother has sole custody of his 2 children, their mother is in the wind, but never signed off parental rights. My brother is married to wife #6 and they just had a baby. They have been living in our home for the past 4 months and my brother accepted a security job in Afghanistan.

My brother is leaving the 2 minor children (and the new baby) with wife #6 for the year while he is gone. She has power of attorney for both he and the kids. I understand that the baby is hers, and I will leave the baby with her. However, I want the kids to stay with me. Wife #6 agreed, but when we told them they would have to pay child support for them, she said no and that she would keep the kids. She is currently looking for a 3 bedroom apartment for all of them, and we have an eviction letter out to them as they have not paid rent and expenses as agreed.

I am worried that wife #6 will not pay attention to them and since this is wife #6, I don't believe she will be in their lives long term. I am worried for their mental stability and the parade of "moms" in and out of their lives.

What rights do I have and how hard would it be to get temporary guardianship/custody of the kids?

Step parents have rights that you won't. The only way you would get custody is via a court order. And with a Stepmother here with a POA, your chances are essentually zero. Unless you can prove the children are in danger.

What rights do I have
You have no rights

and how hard would it be to get temporary guardianship/custody of the kids?
The ONLY way is to prove all their parents unfit by clear and convincing evidence.The definition of an unfit parent is governed by state laws, which vary by state. A parent may be deemed unfit if they have been abusive, neglected, or failed to provide proper care for the child. A parent with a mental disturbance or addiction to drugs or alcohol may also be found to be an unfit parent. Failure to visit, provide support, or incarceration are other examples of grounds for being found unfit.

I agree with smoothy that with a Stepmother here with a POA, your chances are essentially zero.

There is one remote possibility here. Find the mother and have her fight for custody.