Ways to respond to younger sibling asking for help to cheat

How should one respond to a younger sibling asking for your help to cheat? Lets say a high school exam.

How responsible is an older sibling for the younger sibling's decision to cheat?

Where would an older sibling's role end?

I think the first step is to find out why they made the decision to cheat. This isn't to justify the decision, but rather better understand what drove them to this (e.g. laziness, desperation, peer pressure, etc.) so that you can tailor your response.

I feel that an older sibling shares a (small) portion of the parental responsibility: they have the obligation to be a role model, and help with ensuring that the younger sibling(s) is protected from dangers.

The response should be influenced by the relationship between the siblings, the reasons behind the cheating, and the relationship with the parents. First and foremost, though, the older sibling needs to respond with the younger siblings long term interests in mind.

I don't have any siblings, so I can't really speak to the obligation of sibling to sibling, but I do believe that it is the right thing to do to bring the situation to the parents, if this is in the long term interests of the sibling. Sometimes you have to risk hard feelings in order to protect those you love. Which is worse? Your younger sibling feeling betrayed by you, or your younger sibling developing a pattern of avoiding responsibility, lying, and cheating to scrape by life's challenges?