How do I deal with favoritism?

I have two children and as a mother my son always tells me I preffer my younger daughter which is not true is just that he is older and knows better that to get into a fight with his younger sister, he should tell me so that I can talk to her and punish her if necessary but instead it gets worse and bigger I hope that is not your case but maybe you should tell your mom how you feel and hopefully she will see if she is doing something wrong (not that she will admit to it) but at least she can start changing. I hope this helps and remember that communication is very important.