Rubberbands
Some people are just difficult. You say that they have issues with other people as well. This seems to be a pattern with this family. There are people who look for silly excuses to be angry with people. From everything you have described thus far it seems like these people have some sort of social issues which makes them difficult to get along with or which makes it difficult for them to like other people. Maybe Im reading it wrong but this the impression im getting.