The Texas Probate Code defines "interested persons," in relevant part, to be: children, heirs, devisees, spouses, creditors, or any others having a property right in, or claim against, the estate being administered . . ." In order to contest a will, you must be an "interested party."
The interest referred to must be a pecuniary one, held by the party either as an individual or in a representative capacity, which will be affected by the probate or defeat of the will. That means you must have a financial interest. An interest resting on sentiment or sympathy, or any other basis other than gain or loss of money or its equivalent, is insufficient. For instance, if you are a neighbor and you see that an undeserving child seems to be ending up with all of the property of their parents or grandparents, there's nothing that you can do because your interest is not pecuniary. It is just altruistic. The only thing that you can do is to notify an interested party of your concerns. If they are not interested in contesting the will, there's nothing else for you to do.