Nobody is entitled to an inheritance from their parents or grandparents. If you've proven time and time again to be totally irresponsible with large sums of cash in a way that's harmful to yourself and others, I wouldn't blame your parents for giving their life savings to your sister under the condition that she never give you a dime. At the same time, it's understandable to be disappointed if your parents don't include you in their will when you've been nothing but a good child to them, especially if you know that they have sizable assets to spare.
What do you think is an unacceptable reason from withholding inheritance? This story has one. The parents refuse to give their daughters their inheritance if they weigh above 150 pounds at any point, even if they're carrying a baby.