From what I've seen it's more common for the in-laws to cause problems than the siblings.  The spouses of the siblings stoke the fire of friction that carry over to the siblings.  Crystal clear communication between everyone before the death of Ma and Pa can alleviate many problems.    Some lawyers have their Will clients do a video with the client talking into the camera giving their wishes and their concerns.   It can cut both ways if Ma and Pa look confused or not all there.