What should be done upon seeing one's sister with a strange man in an isolated place exchanging words and kisses?

Whoever sees an evil must change it to the best of their ability, as the Prophet, peace and blessings be upon him, said: "Whoever among you sees an evil, let him change it with his hand; if he is unable, then with his tongue; and if he is unable, then with his heart — and that is the weakest of faith." What your sister did is an evil that must be changed, and the necessary means must be taken to prevent her from returning to it again.