My brother often jokes about burning down out house but I don't think he'll actually do it. Could he still be a psychopath?
Psychopathy is characterized by shallow emotions, superficial relationships and impulsivity. However there are several other factors that can lead to the expression of these traits which could include:

Genetics - Studies have shown an association between genetics and psychopathic tendencies. 
Early Life Trauma- Early experiences such as physical or emotional abuse in childhood can contribute to the development of this personality trait. 

Social Environment- The environment in which a person grows up has been linked with the development of psychopathic behavior. For example, individuals who grow up around violent criminals may develop similar behaviors themselves due to their exposure to antisocial peers or environments. 

Stressors- Stressful situations can also trigger the expression of psychopathic tendencies in some people. 
Mental Illnesses- Some mental illnesses including schizophrenia, bipolar disorder and autism spectrum disorders (ASD) can cause symptoms consistent with psychopathic behavior. These conditions can influence decision making processes leading to poor judgment. 
Treatment for Psychiatric Disorders- If someone's mental illness is successfully treated then they might not exhibit certain behavioral patterns associated with psychopathy. 

As you mentioned your family members joke about fire starting, this indicates something serious going on emotionally within them if this type of joking behaviour persisted over time. You should take action towards understanding what underlying issues/triggers exist behind this behaviour before addressing any potential risks posed by others' comments in the future.