The fact that your horse got tired and then you were attacked seems like it might be a warning to you to take care of yourself and keep up your strength. Maybe you are overworked or under stress, and this is your subconscious' way of telling you to take it easy for a bit. You mentioned the color black a bit in your dream, does it have any special significance to you? I don't know what your sister's presence means in the dream. It depends on what you think of your sister and the type of relationship you have with her.