Fibromyalgia is condition that usually affects women. It s physiopathology is not well-known, but there are strong evidence that it is caused from the lack of a neurotransmitter (serotonin). It is believed that it could have a genetic tendency to pass from parents to child (several genes have been found more often in people with fibromyalgia), as it is believed that the way that the body perceive and processes the pain is transmitted genetically. Anyway, until today there isn t any study that can confirm certainly this theory. Some of the risk factors are menopause, poor physical condition, trauma and surgery, emotional stress. So it s more posibble that your mother suffered from this condition herself, and passed it to you, and having a stressful life may have predisposed both of you sisters develop the condition. Regarding the Arnold-Chiari concern, it is caused by a congenital malformation of the skull, wich causes the cerebellar tonsils to descend in the foramen magnum or below it. It s prevalence is 0,1 - 0,5% ( wich means 1 to 5 children in 1000 births). There isn t any available evidence of genetic tendency, so it s unlikely for you to have this condition too. I hope i have been of help.