Since he trusts me and asked me, it's best for me to teach him the most important things I know about Bitcoin and explain the risks. Then, let him make the decision himself.
You've done the right thing by providing them with the education they need to know, so they know what to do and what the risks and benefits are.
If you don't do that you will probably feel guilty when your siblings log into bitcoin without knowledge and they end up getting bad things when they start logging in, this will be a bad impression.
When you have provided basic knowledge and what to do and what not to do, then your task is done.
You only need to decide whether to start investing or not, it is a personal decision.
But in my experience you will still be tied to your family when one of them suffers a loss in crypto,
because you are the foundation as someone who already understands crypto beforehand.