1) Yes, stockpiling money (stocks actually) is the thing to do. This is essentially a liability hanging over your heads, so when you're wanting a bigger car, or a restaurant meal, collectible/recreational stuff, or a fancy vacation please keep this potential liability in mind. If your worst fears never happen, the only downside is you retire rich. Acting decisively and early here will give your assets the best opportunity to compound before the expenses hit.
I strongly do not recommend getting a duplex, because that's like rolling out a red carpet for in-laws who you are afraid will exploit you. I wouldn't rent to functional family members, much less dysfunctional ones. That's a guaranteed recipe for no rent and an even messier situation!
2) Maybe taking an interest in helping the parent-in-laws with the SIL with Down Syndrome could open up a relationship and help you find out details like what programs she's been signed up for, if they've looked into various programs or local educational options, if they've talked to a social worker at your state's Medicaid office, etc. Similarly, with the other sibling, are they getting the treatment they need, what programs are they signed up for, etc? This will help you quantify the issue. Right now it's a little scary because so many of the details are fuzzy.
I don't know the details but early intervention could make a big difference with either sibling. People with mental illness do routinely succeed in treatment and become functional, productive adults - and how awesome would it be to have an ally on your side in a few years? Or even a babysitter? Similarly, people with Downs can learn occupational skills and earn much of their cost of living - but they probably need to be in the right kind of school right now. One of the sanest people in my family is someone diagnosed with paranoid schizophrenia who after years of treatment arrived at the correct pharmaceutical and lifestyle options, and is now a trustworthy and reliable babysitter for us.
And... who knows... maybe as you take a bigger role the parents open up to possibilities such as financial / mental health counseling for themselves? Imagine your relief if they started real jobs a couple of years from now? For all you know they're tired of falling for the hype too. You just have to start the conversation with little offers to help with the SIL's and then get closer over the course of many months. To start, have them over for dinner and ask if there's a way to help with little administrative chores.
3) The problematic thing is you've tried to change their behavior before. So they're going to be on the defensive, and will be on guard against ever being told "I told you so!" or having their own failures rubbed in their face. You'll have to cease the advice-giving / pleading approach immediately if you are to accomplish the scaled back goal of obtaining info about finances, education, or healthcare for the siblings. Your role is only to help with little chores like driving to doctor visits, arranging for taxes to be done, talking to social workers or service professionals, and relieving the burden on the parents. In doing so, you achieve your goal of steering this situation in a less tragic direction.
To get into this mindset, you'll have to let go of the fantasy that the parent-in-laws will someday get their shit together to relieve you of the burden of worrying about the sibling-in-laws. It would be best if you assumed that will never happen, and to steer the relationship in a direction where the parents do what they can, and you do what you can, and the siblings get what they need without all the burden falling on one couple (either you or the parents).
