Do I need a co executor's signature on tax documents to file them for the estate?

Do I need a co executor's signature on tax documents to file them for the estate? My sister and I are co executors of my father's estate. When he was alive I was his power of attorney and took care of him through his illness. Since I was his POA, I had his mail forwarded to me with my sister's permission. I took all of his tax materials to a tax preparer who sent a copy certified mail for my sister to sign. She would not sign for the documents and they were returned to the tax preparer. Do I need her signature on his tax return or can I sign as his personal representative? In general she has been uncooperative. I have been maintaining my father's property and paying his outstanding bills out of a small amount of money we agreed upon after his death. But I am frustrated that she will not cooperate in handling his affairs and will not accept the tax documents for signature. Do I need her signature or can I sign as his personal representative and send her half of the refund sent to me? Please help