Yes they are in the same family as the Black Widow, which makes them neurotoxic - the female also eats the male while mating. Yikes!
Bites from Redback spiders produce a syndrome known as latrodectism. The symptoms are similar to bites from other Latrodectus spiders. The syndrome is generally characterised by extreme pain and severe sweating. Initially the bite may be painful but sometimes only feels like a pin prick or mild burning sensation. Within an hour victims generally develop more severe local pain with local sweating and sometimes piloerection (goosebumps). Pain, swelling and redness spread proximally from the site. Systemic envenoming is heralded by swollen or tender regional lymph nodes; associated features include malaise, nausea, vomiting, abdominal or chest pain, generalised sweating, headache, fever, hypertension and tremor.[13][14] Rare complications include seizure, coma, pulmonary edema, respiratory failure or localised skin infection.[17] Severe pain can persist for over 24 hours after being bitten.[18]