Learn Danish: Bortset fra svigerinden er der ingen der kender hemmeligheden. - Apart from the sister-in-law, there is no one who knows the secret.

You can, but bortset fra is the most natural choice when placing the exception at the very beginning of a sentence. It works exactly like "apart from" in English to set up the context before introducing the main clause.

Why is the word order er der rather than der er?

This is a classic example of the V2 (verb-second) rule. The entire phrase Bortset fra svigerinden occupies the first position in the sentence. Therefore, the main verb er must take the second position, pushing the dummy subject der into the third position.

Why is der required after ingen?

Here, der is a relative pronoun meaning "who". In Danish, when a relative pronoun is the subject of the relative clause (the one doing the knowing), you must use der or som, and it absolutely cannot be omitted like it sometimes is in English.

How does the word svigerinden break down?

It consists of the prefix sviger- (meaning "in-law"), the root inde (a female suffix, making en svigerinde a sister-in-law), and the definite suffix -n to mean "the sister-in-law".