Is rheumatic fever something like sepsis?

Autoimmune. Rheumatic fever is a sequels of untreated streptococcal throat infection. Body produces antibodies to the bacteria which can attack joints and heart valves as they share some protein structures with the bacteria.
Quite different. In sepsis living germs are circulating in your blood, triggering changes in blood pressure and attacking your organ systems. In rheumatic fever, antibodies created by your own immune system long after a strep A illness begin attacking various organ systems. (Heart,brain,kidney). The only similarity is you are quite sick.