If you can, you might consider talking to an attorney about a trust. I had legal insurance as a company benefit once (highly recommend it if you have an option) and it covered wills and trusts. I had legal documents drawn up outlining how money would be distributed to my minor children/their caregivers/etc. There are a lot of ways to set up funds so that their spouses can’t touch them, so if you can get in touch with a legal advisor or go down a good old fashioned internet rathole. Good luck and what a wonderful aunt you are!
I'm single and childless but deeply concerned for the future of my nieces. I don't think it's too far off to think we may see a future where we return to what you're talking about here. I plan on leaving what little bit of wealth I've amassed to them and can only hope they put it to wise use after I'm gone.