does the older child try to influence the younger one? can the older child be spoken to to try and help them see that they’re influencing the younger one?
Also why is the older one “messed up” and why are you calling it “messed up”? anyone thats going through stages is certainly not messed up – maybe they have questions that need answering. and maybe you should focus on helping the older sibling rather than focusing on the damage they’re doing to the younger one.
if the younger sibling just sees you blame the older one for the influence it will only make the influence a bigger yetzer hara. specially if the children have something against whoever is trying to teach/educate them.
but if they see you try to help the older one they’ll see that this isn’t the normal since that person needs help, and will not necessarily follow…