I would be happy to help assist you in understanding your dream. You mentioned your having difficulties in your relationship? The black cats are symbolic. #4 is the number for creation. Black usually represent negativity or evil. Sofa represent support.
I believe the cats are a representation of your spouse. Since the cats did not attack your sister but only you. Perhaps you feel that you are being "attacked" by your spouse. When you defend yourself or fight back it only causes your spouse to become more angry. With saying the lords prayer and the cats disappearing is showing this is a guidance dream on what you need to do in your current relationship. God is showing you that you need to go to him during these difficult times in your life. When you get into a disagreement or argument with your spouse. It is best to step away before coming angry and have a conversation with God about what is taking place. Take your anger out on God, speak to him. You will be amazed by how he works. It may not happen over night, but choosing whether you want to fight for your relationship, continue to do this and you will see amazing changes take place. I myself have had to do this in my marriage. It wasn't easy, but one does not reap a harvest over night. If you have anymore questions please feel free to ask.