I suspect that any claim by parents, grandparents, aunts/uncles, or cousins that they were unintentionally overlooked in your will would be met by the court with a high degree of skepticism.
However, if you want to be ultrasafe, you can certainly exclude people by name. But, a provision in your will along these lines should be sufficient:
The omission in this will of any provision for any other person or relative is not due to oversight or neglect, but is based upon my desire to benefit only the beneficiaries designated herein.