My CT scan of the abdomen and pelvis showed sclerosis on both sides of the SI joints, indicating sacroiltis. Is this indicative of multiple sclerosis?

Of course not. "Sclerosis" is a general term that refers to pathological hardening of tissue due especially to calcification or overgrowth of fibrous tissue. In the CNS, where there is no fibrous tissue, it refers to an analogous process of tissue degeneration. Multiple sclerosis by definition occurs only in the CNS, not in joints. This sort of information is readily obtainable from a quick web search.