My sister says she has pericarditis. are family members more likely to get the same thing?

Probably not: The usual cause of pericarditis is a virus and/or is "idiopathic" (doctor talk for we don't know the cause). These do not affect family. Rarer causes are cancer, TB or bacterial infections, also unlikely to be familial. Some autoimmune diseases may run in families but are very uncommon. So, in short, almost certainly not a worry.