Thanks for the longer explanation. I definitely see how the special needs can change a lot of mustachianism decisions. I don't like the assumption that the two kids will spend their lives taking care of their special needs sibling. That's not a burden they chose, especially the unborn one! And no one has addressed the possibility that the third child also has special needs, which is not uncommon with things like autism where there is a genetic component. The car demands simply don't bode well. Any family can come up with a list of things they need to make their lives better, but the essence of mustachianism is the sense of enough. I grew up poor as well, which makes it easier for me to say kids growing up now have a wealth of opportunities via the computer and Internet. You can diy music lessons or learn about art, math, science, or learn to cook or fix your car, all for free. Nice vacations and enriching experiences are frosting on the cake - if you earn enough to afford them, I dont judge, but why does anyone under 18 "need" a fancy vacation? What are they vacationing from, and wouldn't a camping trip or other outdoor adventure be adequate?
Eta: I think in this situation I would be even more inclined to set financial limits on brother and try to come up with a long term plan for special needs nephew. I suspect SO will be the long term financial support, not nephews siblings, and you need to decide how comfortable you are with that.