The best thing that you can do is love them unconditionally. There is a huge difference between “tough love” and “loving tough” so do your best to “love tough” as tough and as hard as you can so that your sib can feel it with every nerve ending in their body. They have to feel your love and your care over and above the vibes they are getting from the friends they are making that are accepting them for who they are and are validating their pain.
Try to be the shoulder they cry on, if you can be the ear they vent to, it can keep them from joining others who are OTD and help them on their way. When I work with At-Risk kids I woulds say “If a goy hurt you like this Yid or Rebbe hurt you would you say ‘All Goyim’ are bad? Why do you lump the whole religion together and take out your frustration on the entire religion instead of just this one person with bechira who gives in to his yetzer horah instead of listening to his yetzer tov?” That really gives them something to think about. I also say “Gie it over to Hashem because he is the best score keeper and believe me, he DOES keep score. He rights everything down, everyone has their own page and he writes everything down, and after 120, he will have to give a din v’chesbon and answer for very kid her hurt not just you. Every single thing he said and every single thing he said. Him and every other Jew who does not live up to Hashem’s true Torah Values”
People are only human, and they have the same human qualities as do goyim. Everyone makes choices and whether they are a Rebbe, mechanech, teacher, parent or child, WE all make mistakes. Unfortunately some of those mistakes hurt other people and sometimes to such a degree that it pushes kids OTD. If we can show that it has nothing to do with Yiddishkeit but with human nature and personal mistakes and choices, then we can save kids from running away from who they really are.