my brother died within 1 hr of arriving at a nursing home. I was poa in his life. I did not sign any admission papers and now they want me to sign poa

They can do nothing.  You are correct that when your brother, the principal, died you lost all authority as agent under his Durable Power of Attorney.  Beware of the old trick of trying to make a family member sign as "responsible person," which makes you personally responsible for the bill.  Also understand the nursing home's incentive:  Medicaid pays them about $200/day but $800 for someone with covid-19.