I'm so sorry, Joanna. You should not have had to endure that harsh treatment as a child. Although I don't approve of your mom's favoring your nephew over your children, be glad of the blessing in that. Your children should be free to grow up in the unconditional love you will give them. Your poor nephew will have to live up to some steep expectations, and if that falling out comes, he may be extremely hurt when she drops him.
Continue to reach out to your siblings without your mother being gate keeper.