If Saudi Arabia is a sponsor of terror, the proper avenue of redress is diplomatic or military, i.e. the President's use of his powers to freeze Saudi Arabian assets and create a fund to compensate 9/11 victims. This is not an issue that should be adjudicated by the courts.
Also, I would like to mention that that the 9/11 victims are the best compensated victims in the history of terrorism, and many of them have become millionaires.
What Have We Learned About Compensating Victims of Terrorism? | RAND
Over 97 percent of those eligible voluntarily applied to the fund rather than choosing to litigate against the airlines, the World Trade Center, the Port Authority of New York and New Jersey, the Massachusetts Port Authority, and others. The average tax-free award to surviving family members was more than $2 million, with physical injury awards ranging from $500 to $8.6 million for a survivor who had suffered third-degree burns over 85 percent of his body. By the time the fund terminated on June 15, 2004, it had awarded more than $7 billion to 5,553 eligible families and victims.
Victims of the Oklahoma bombing, by comparison, did not receive this outflowing of generous charity.
Also, I would like to point out that the survivors of your typical personal injury victim does not receive an award as high as $8.6 million, even for burns covering the entire body. While this doesn't seem over-compensation for an injury this severe, most victims must settle for less because a commercial policy has $1 million per person/$2 million per accident limits, and personal policies have even lower limits.