The estate tax is imposed on the transfer of the "taxable estate" of every decedent who is a citizen or resident of the United States. Accordingly, assuming you meet the domicile residence analysis test, you will be subject to the same estate tax rules on your world-wide assets the same as a U.S. citizen. Since your parents are now naturalized U.S. citizens, they are also subject to the same rules.
Thus, both you and your parents are entitled to the current $2,000,000 exemption, however, while your parents are entitled to the benefits of the Sec. 2056 marital deduction (i.e. assets passing from the decedent spouse to the surviving spouse are not subject to estate tax) you as a resident, non-citizen are not. If your spouse's estate exceeds $2,100,000 (i.e. amounts transferred to a resident, non-citizen spouse in excess of $100k are not eligible for the marital deduction), then that excess will be subject to estate tax on his death unless you implement a qualified domestic trust (QDOT).
Generally, this trust requires at least one trustee to be a U.S. citizen or corporation who will be responsible for ensuring that all taxes due are paid prior to any assets leaving the country. The trust must authorize the trustee to withhold all required taxes from any distributions. Also, only income may be distributed unless the trustee withholds the requisite taxes from any principal distributed. Finally, the trust must be sited and administered under the laws of one of the 50 states or D.C.
Conversely, any assets you leave to your U.S. citizen spouse will be eligible for the marital deduction. So the only event you really have to plan for is if your husband pre-deceases you and transfers assets in excess of $2,100,000 to you.
Because it is impossible for me to identify and consider ALL the relevant facts, this advice is not intended or written to be used for the purpose of avoiding penalties, and cannot be used for that purpose.