I don’t know if your parents have made a will, but if not, you should encourage them to take that step. Not only will it help avoid legal difficulties later on, but it could also make clear your parents’ true wishes.
At the same time, a will probably won’t solve all your family’s problems. The reason, I suspect, is because your brothers and sisters are acting from motives that have very little to do with what they hope to inherit. Some, admittedly, may be motivated by greed, and they hope to enrich themselves. Yet greed is very destructive; the Bible warns that “A greedy man brings trouble to his family” (Proverbs 15:27).
Others, however, may argue simply because they see their brothers and sisters as rivals, and are constantly trying to get ahead of them. They also may be driven by jealousy or anger, and don’t want anything good to happen to their siblings. The Bible says, “An angry man stirs up dissension, and a hot-tempered one commits many sins” (Proverbs 29:22).
Whatever their reasons for arguing, ask God to help you be a peacemaker–both by your words and by your example. Go out of your way to avoid arguing, and to show the love of Christ to your siblings and your parents. Above all, pray for them, and ask God to convict them of their sins and draw them to Christ.