Cat behavior problem.

We got a 13 year old girl recently (just months ago, not even a year). And we have a 3 year old girl named Po that we have had since she was months old. She has always been mean to the new cat and has always bullied her too, but it isn't that Po is just mean because we also have a 9 year old male and they cuddle and lick each other. At first we thought she would quit bullying after she got used to the new cat, but it has just gotten worse. Po will actually go and seek out the 13 year old where ever she is hiding just so she can attack her. We put the 9 year old and Po in the basement during the night and leave the 13 year old up stairs so they don't fight during the night. We have tried just leaving them alone to sort it out themselves and we have tried separating them. No luck. Our 13 year old cat is living in fear and is too scared to come out of hiding most of the time. I'm affraid they just can't live together, but if there is anything that could help, please tell. Why do you think Po is so insistent on being horribly mean to our 13 year old, and is there anything we can do to help stop the bullying and attacking?