If they are there with your father's permission, then no, you can't remove them from the home. Do they live there? If they are residents, then the owner of the house or their representative has to go through the correct legal ways to force them to move. This can take some time. Is your father competent? Does he want them removed?
Are the nephews caring for dad? Are they paying rent? Have you asked them to leave?
Do you have Durable Power of Attorney or simply Medical Power of attorney?
Does Dad want the nephews to leave? Is there financial abuse?
Question 1 - Is your father of sound mind?