The Cat Who Tailed a Thief: Lillian Jackson Braun

After reading a few of these books it becomes obvious on who the culprit or caper will be. Someone new comes into town and soon someone ends up dead. In this on a "brother and sister" move to town, they both marry wealthy people that soon mysteriously die. The woman also has a problem with kleptomania which may have been a way to divert suspicion from the murders. Polly, Qwill's girlfriend, loses her last living relative to the couples murdering rampage.