Is it legal for an heir to request their parent to change their will if the heir does not like the spouse of another heir?

There are 4 heirs. One is also the executor. Noone knows the terms of the will or who the executor is except the writer & probably 1 of the heirs. One of the heirs does not like the spouse of the executor/heir and tells her parent to change the will in their favor. The rest of the heirs, including the original heir/executor, have assumed since the beginning that the heir who requested the change has been the executor from the start. The willer/parent followed through on the request, and added a codicil to the will changing the executor. The willer is of sound mind & body.