I am a law school graduate. What I offer is mere information, not to be construed as forming an attorney client relationship.
Okay, if both parents died and you suspect the brother and sister were both named as beneficiaries, this is what happens:
with or without a will, life insurance is probably probated. Other parts of the deceaseds' estates were probably probated.
Probate is public information. Go down to their county records office, and get the information. Since you do not know the name of the life insurance company, you will probably need to hire an estate attorney to do an investigation and CONTEST THE estate.
Try attorneypages.com or contact your state bar association (411) and ask for lawyer referral.