Is it legal for me to own a taser gun?

I live in San Francisco, CA and I would like to get a taser gun to defend myself in case someone breaks into my house. I have a druggie sister and her boyfriend is black they're planning to harm me cause I called the cops on them when he hit her and she defends him,... now they're planning to do something to me. I told the police but they said they cant help me cause I dont have proof so I want to know if its okay for me to own a taser gun incase they break into my place. I dont live with my parents.. please let me know!