Is refraining from advising a sister who commits Sharia violations in her dealings with her fiancé, after she and her father rejected the advice, the correct action?

The suitor is a stranger to the fiancée as long as the legitimate marriage contract has not been performed. His interaction with her is like that of any other non-mahram man. Therefore, he should not be alone with her, nor should he speak to her without necessity. The father must prevent his daughter from exceeding the limits of the Sharia in her dealings with her suitor. The questioner should continue to advise his sister and father gently and remind him of his responsibility.