Thank you for your question. I look forward to working with you to provide you the information you are seeking for educational purposes only.
Under VA law, absent a will to the contrary, without a surviving current spouse, your brother's children are the sole heirs to his estate with rights to that estate. Legally, even as executor of your parent's estate, you do not have a right to see what is in your brother's estate. If there was perhaps an insurance policy with your brother as beneficiary and your parents as insured, that would still go to your niece. However, typically, without a will, she gets his whole estate (presuming she is only child, which is easier for now, but you understand if he had multiple children the same applies and they share their father's estate equally).
If she has something that she needs from your mother, as you are executor, she needs to make the request to you in writing. If you choose not to provide it you can force her to go to court to get a court order (since we have no idea why she needs it I could not even guess).
As estate administrator, you do not have the right to know anything unless there was something that was supposed to go to your mother's estate. It is likely your brother may have had an insurance policy or something naming your mother as beneficiary, but if the beneficiary dies before your brother, then that beneficiary grant is void and whatever it was granted to your mom goes into your brother's estate, it would not go to your mother's estate because she died before your brother.
If you refuse to provide the death certificate, she can actually petition the court for an order to get a copy from the department of vital records without your consent and the court will grant it.