You're doing the right thing. I would gently encourage and share information, and have conversations with them when they initiate or when the actively respond to very gentle initiation on your side. That along with a 50% match without further restrictions if you want a tangible incentive.
You don't want your relationship to be one where you're the hectoring relative who comes across as a Scrooge obsessed about money all the time. It is far better that they see you as a positive role model they would like to emulate.
I know your intentions were the best and from love and concern for your nephews, but the initial proposal looks like it was written by Congress .