Nowadays there would be a sequel (Spoiler warning)
Considering the ending, obviously there was more story to tell.
Back then sequels weren't made often as they are today.
Considering it wasn't a box office success, a sequel was probably never considered.
The only way a sequel could have worked, even semi successfully, was to be more creative with the next set of killings ... but how could they top killing off a baby?
Another set of twins?
Possibly had they used the same set of twin actors perhaps 5 years later in 1977 showing what they were like a little older.
However, by then we had "The Omen".