it's like this :Rachel cannot date other people because she is still in love with Ross.But she didn't know it.She thought she'd get over that relationship already.
So Pheobe ask if it's the relationship with Ross that hinders Rachel from getting a new BF,Rachel said no (actually yes,but she thought no)
So pheobe asked if it's the relationship between rachel and her father.See,it's a humor here.It is not possible her father has anything to do with the fact thay rachel cannot date