Here are the differences between "their, there, they're " even though they are pronounced in the same way http://www.ehow.com/how_2262043_difference-between-there-their-theyre.html

Oops. I think I understand your question now. I don't think we call it a word family. However, in your example, the part of the word "develop" is called the "stem". Like the stem of a flower. All the different endings are like the leaves. So, in the list of words you provide, they all have the same stem. Words with the same stem - is that what you are looking for?

I suggest you consult a dictionary. http://www.thefreedictionary.com/develop their (t̸her) possessive pronominal adjective of, belonging to, made by, or done by them: also used before some formal titles [Their Majesties]: often used in connection with a preceding singular pronoun (as everybody, somebody, everyone): did everybody finish their lunch? their (Þâr) adjective The possessive form of they. 1. Used as a modifier before a noun: their accomplishments; their home town. Webster's New World College Dictionary

I am not too sure whether there are rules for this stuff or not. There probably are, but it won't help you to learn a list of rules. I suggest you learn these words and figure the rules out for yourself. Most of the time we make nouns from adjectives. Try this: http://fcegrammar.wordpress.com/2007/02/27/nouns-made-from-adjectives/