Is it common for an estate attorney to represent the executor of the estate (who also happens to be a beneficiary) and not the other beneficiaries?

The attorney for the estate technically represents the executor.  Both the executor and attorney have fiduciary duties to protect the interests of the beneficiaries.  In a typical probate, the beneficiaries do not need counsel because the executor, the attorney, and the court are making sure that the testator's wishes are carried out and the beneficiaries receive what they are entitled to receive.

If your situation is not proceeding this way, you may wish to consult an estate attorney to find out what you can do about it.