To what extent can a family member insist on helping a brother/sister to stop misusing substances when there's a resistance on getting help? Are relatives the most adequate in enhancing change?
You ask two questions
1. What extent can you INSIST?
and the answer to that is practically not at all
2. Are relatives the most adequate in enhancing change?
and the answer to that is they certainly have the proximity and motivation, so they might not be the MOST adequate, but they can be adequate
You should think about changing your stance - stop thinking about insisting and start thinking about influencing. Google "how to persuade and influence" and do some research on things like active listening, negative physcology (I won't put any links up as some sites are selling books).
There are lots of 'tricks', and I don't know what is most appropriate for your situation, but for instance a 'trick of the trade' for parents is to get their kids involved and support them in sporting activities to 'keep them out of trouble', it gives them a meaning and some goals in life beyond self destruction.