Is the title "二寡妇" synonymous with prostitution?
Short answer : No.

Longer answer : 

In Chinese  traditional culture, one follows a hierarchy called relatives circle 伦理. Relatives never call each other name name directly among themselves, instead, Chinese use ordinal and the person hierarchy and relatedness according 
base on one to another.

For example, if you have two elder uncle call Tom (1st) and Jerry (2nd), your father is the 3rd, two younger uncle call John (4th) , Nathan (5th). This is how you call them 


Tom 大伯 （Eldest uncle ,伯 indicate he is elder than your father)
Jerry  二伯 （2nd uncle)
John 四叔 (4th uncle)
Nathan 五叔  （5th uncle, 叔 younger uncle than your father)


The society usually don't know each of your uncle name but they know each family  relatives circle hierarchy.  I.e. Say your family clan name is 白, this is how the society call your uncle (ordinal and clan name) and their wife


Tom 白大  , wife 白大嫂  
Jerry  白二 , wife 白二嫂
John 白四 , wife 白四嫂
Nathan 白五 , wife 白五嫂


When Jerry pass away, his wife can maintain the relatives title(嫂) or called as widow(寡妇), then 白二嫂 can be also be identify as 白二寡妇 by the society.