Is prommissory note viable after 9 years?

I'm not sure why the promissory note is no longer viable.  The statute of limitations on enforcing a promissory note runs from when it is breached, e.g. when payment is due and not made.  Thus, if the note was executed in 2006, but wasn't supposed to be paid until 2011, the statute of limitations would start to run in 2011.  If payment wasn't due until demand, the statute wouldn't start to run until demand for payment was made and not honored.  If no demand was made, the statute would not have begun to run, and the fact that the note was executed in 2006 would not prevent your mother's estate from enforcing it.  Thus, assuming you didn't pay a $100,000 demand promissory note on which demand was first made in 2013, you have 3 siblings, and all four of you inherit equally.  Your mother's estate can sue you for $100,000 (plus interest, and attorneys' fees if the note so provides.)  If  you are found liable (you may have other defenses), you would have to pay the estate $100,000 (rounding off), which would then be divided four ways (after paying off the estate's lawyers for their services in suing you) along with the rest of the estate.