Three-Headed Tower (1956)

Newly graduated Miyamoto Otone is to inherit a fortune from a distant relative on the condition that she marries Takato Shunsaku, a man she has never heard of. The lawyers haven't located Shunsaku yet, allowing Otone to think about whether she accepts the conditions of the will. One month later after, Takato Shunsaku is found. Murdered. At the birthday party of Otone's uncle. With Shunsaku dead and thus making it impossible for Otone to marry him, the inheritence is to split amongst all (living) family members. And yes, as always, that means that the potential successors get killed off one by one. But Kosuke Kindaichi is on the trail...