Answer: The elements of the sentence need to be relocated.
Explanation:
In English grammar, the basic structure of a sentence is:
SVOCA (subject, verb, object, complement and adverb)
The mistake in this sentence is the location of the adverb clause. They could be not only at the end of the sentence but also at the beginning. That is, the correct version would be: "At the beginning of January, my sister had twins and decided to quit her job".
However, if we were to put the adverb clause at the end of the sentence, the meaning changes completely. "My syster had twins and decided to quit her job, at the beginning of January". This means that both actions occurred at the beginning of January, while the first sentence means that she only had twins in the first days of January, but we do not know when she quit her job.