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It depends upon the rank. If your parent is a king or queen, they are all princes/princesses, and may also hold the titles of duke or earl of other lands. If your father is an earl or other nobility and you're the first son, you will become the earl and your siblings will be simply Lord or Lady. They are still considered nobility - just not high ranking.
Queen Elizabeth's youngest son recently had a child and his child will not hold a royal title. That is done by request of the parent. He is still part of the royal family. His son is a Viscount and his daughter is simply known as Lady. I believe also that Anne's children are not ranked. Usually the female grandchildren take their social rank from their father, regardless of the fact that their grandmother is the Queen.
The order of rank in English aristocracy is: King, Duke, Marquess, Earl, Viscount, Baron.