Can my brother stop the planned cremation and pay for a burial, with his money, even though I was the beneficiary and had already planned a cremation?

Did the deceased woman specify what SHE wanted ? If she did not want to be cremated , I think I would try to honor her wishes , since your brother is willing to pay for the more expensive option .

whomever is the administrator or executor is the person who will arrange funeral services after death.
A beneficiary on a will does not decide the funeral services. The executor does. Sometimes the POA has pre-arranged to have this done according to the instructions of the decedent, also.

Please could you make it clear what you mean by ‘I was the beneficiary’. Did your mother leave a will and who is the Executor? Were you the only beneficiary under her will, and is that why your brother wants some control? Why is cremation versus burial an issue between you and your brother? There are some religious sects that think the body being kept whole is important for the after-life – is that the problem? Was there a pre-paid burial insurance policy, or has a plot been prepaid? None of this is at all clear, so we can only guess.

If I understand correctly you were the benficiary on her life insurance policy? Who is the Executor of your Mom's estate? Is there anyone with legal authority? If it's not your brother, then no, he can't stop whatever plans your Mom or the Executor or the person now legally in charge in place. We can't answer your question without more accurate information.

Beneficiary of an insurance policy? If you were POA that stops at death. If brother wants to pay for a funeral, why not?

If there was a beneficiary then you're talking about a pre-paid burial policy. In this instance it is the funeral home or crematorium that is the beneficiary and not anyone else. What were her wishes (burial or cremation)? Does he want to pay for a burial because he disagrees with cremation, or your Mom disagrees with it? The policy was either purchased by her, or her PoA, or her legal guardian. If your brother is none of these people he needs to butt out. Ultimately the Executor of her Will will carry out HER final wishes in terms of what happens to her body after she passes. If she never expressed anything, then the Executor can make the final decision (and sometimes it is based upon funds left in her estate to cover the costs).