How is fibromyalgia related to arthritis or autoimmune conditions?

Indirectly. We don't understand fibro well. It's hard to study. No simple blood test, biopsy result, imaging finding or genetic testing confirms dx. There are criteria, but they read like checklists of signs and sx. Often an intial trauma. Pts have higher rates of allergy, so many in my practice. Relationship? Many shared sx: pain in/around connective tissues + systemic c/o: fatigue, fog, can't sleep... .
Multifactorial cause. Fibromyalgia is a multifactorial consequence of hormonal, immune system and nervous system dysfunction resulting from genetic predispositions and dietary and environmental insults. These factors impact your immune system and can lead to pain and inflammation. If affecting your joints-arthritis, if affecting your connective tissue and muscles - fibromyalgia.
Not genetically. Do arthritis & autoimmune disorders relate, etiologicaly, to fibromyalgia, FM. No genetic, but are behavioral relationships. Multiple co-factors conjoin, in varying degrees, to cause FM: prolonged sedentariness is common; leading to soft tissue deconditioning & obesity; inciting widespread soft tissue spasms, ischemic tender points & interrupted sleep; leading to depression, fatigue & fibo-fog. .