Challenging someone's estate plan can be very expensive and very difficult to do successfully. Therefore, challenges should only be made after careful consideration. They should not normally be made merely because you received less from the estate than you expected.
You should instead first consider whether the distribution of assets would seem reasonable to an independent observer and whether there were any suspicious circumstances under which the estate plan was made.
Sometimes that initial inquiry will lead to an obvious answer, as was the case in a recent Market Watch column "My stepfather left $1 million to strangers and dead people."
A Market Watch reader wrote in to ask about some instructions his stepfather had left concerning a trust written a few days before he passed away. Instead of leaving any money for his wife of 60 years or any other close relatives, the stepfather decided to leave everything to relatives who lived overseas with whom he had no contact for years.
At least one of those foreign relatives was dead at the time.
While that is a pretty simple case of plans that do not on their surface appear reasonable to an independent observer, most cases are far less straightforward. Before deciding to challenge the estate plan of a relative, contact an estate attorney and see whether you have a good case.
Reference: Market Watch (Nov. 26, 2016) "My stepfather left $1 million to strangers and dead people."