On top of the maternal influence, there is the tendency among American Catholics to retain a strong ethnic identity in RC parishes.
Parishes are where you're from in most eastern and midwestern American cities with large RC populations, not neighbourhoods. Even little towns are divided into Irish / Italian / German parishes. There is a certain small town in Nebraska that has a St. Patrick's church and a St. Joseph's.
Not that long ago if someone in Chicago asked you where you came from, your answer - the name of your parish - would tell whether you were of Italian, Irish, Polish or German descent.
Members of different parish tribes generally didn't mix. A marriage between members of Italian and Irish parishes was considered a mixed marriage.
The distinctions sprang up because of the tendency of immigrants to live close to their fellow countrymen. Even still there are areas of American cities with a strong ethnic flavour.
Biden was most certainly brought up in this sort of culture.