POA for my father was my sister. she passed away before my father and she gave POA to her 2 adult children 23 & 25. What rights do I have?

Your sister could not give power of attorney on your father's behalf to her children. Only your father could have created that power of attorney. It is possible that her children were nominated as successors when the original POA was created by your father; but it is not possible that your sister herself gave them this responsibility.

In any case, now that you have also lost your father, which I am sorry to hear, it doesn't matter because any power of attorney ended at that point.

Your father died intestate. If you are sure that was the case, look up "intestacy in Kansas" and take your research from there. This link -

https://www.kansaslegalservices.org/node/1092/frequently-asked-questions-about-probate

- may be helpful in explaining the process.

I am sorry that you are not on better terms with your niece and nephew, not even to the extent of being able to pick up the phone and have a business-like conversation with either, but as your father died intestate it is unreasonable to hold them responsible for whatever is happening to his assets. They cannot be his executors, because he had no will; and four months is really not very long when it comes to sorting out the affairs of someone who left no instructions, I'm afraid.

Your nephew is now living in your father's house, is that right? And was he, or was his sister, your father's primary caregiver? Did you have any involvement at all in your father's life?

I do understand the tension and frustration of feeling that you are being kept in the dark, and it can make one very suspicious too. But do try not to let it turn you into the sort of person that on sober reflection you would not want to be.

They think I'm stupid and must not believe or know that I'm a Criminal Justice Studies graduate and think I'm going to go away quietly. I am very tempted to make arrangements to kick my nephew out of the house that's mine (since I'm the next and only kin and sibling). And sell everything that I can.

My father didn't have a Will and I don't know if my sister had one. My niece is the oldest and me and her don't get along and my nephew both aren't giving me answers and u know how young people are these days, they text and no communication whatsoever. I need to know how to go around them to find out how to get the information I need.

Your rights as far as any money goes depends upon the wills if they each had one and someone knows where they are. 

It is likely that your sister's estate will be inherited by her children and that her children and you may benefit from your father's estate. 

Who is the executor of the two estates and have you contacted them?

Your sister could  not give POA to anyone for your father. Only your father could assign POA for himself.
As DeeAnna said, POA responsibilities are over when the agent dies. 

I am sorry for the loss of both your Sister and your Father within 1 year of each other. Since both your Father and your Sister, who was his POA, have passed away; both of their finances should be in Probate now with their Executors managing their Wills and their finances. The responsibilities of their POAs ended when they died. Who is the Executor of your Father's Will and of your Sister's Will? Talk to your Nieces and ask about the Wills and ask who is the Executor for each person? Ask for the name of the attorney who is processing the Probates for your Father and Sister.