What a heartbreaking situation for everybody involved.
I think the age difference might be relevant, from your parents' perspective, OP, as you were already an adult (just) when your DSis's childhood was cruelly ripped from under her when it was only half way through.
Of course, you don't want to feel abandoned by your parents as soon as you reach 18, but from their pov, you're an intelligent, capable, independent adult carving out your way through the world, whilst your DSis is a perpetually highly-dependent child. Purely logically, she needs them 24/7, whereas you don't strictly need them at all - so I suppose they've addressed the need that exists to the detriment of anything else.
Not asking for any details at all, but might they somehow blame themselves at all for the life-changing accident - and thus see it that they've paid their 'parenting debt' to you, but will never be able to do so with her?
I know it doesn't help you at all, but I can see how your own major successes in life would weigh hard on them - ironically more so than if you'd ended up a layabout ne'er do well; they always looked forward to seeing their two children succeed and achieve the significant adult milestones, and whilst you yourself are naturally proud of how your life has unfolded - and you obviously can't live anybody else's life - they see each of your proud moments as a slap in the face to them, as a reminder that it will only happen once to their children, and not twice as they had always expected. To put it bluntly, they see 'our children' as a unit and even if you perfectly aced every single moment, that would only ever be a maximum 50% 'happiness score' for them as parents.
Not wanting to sound callous at all, but I think, at some point, you will need to ask them what their long-term plans are. They'll be realising that they aren't getting any younger and might already have been making plans for you to take over from them after they have gone, or before as their health and physical capacity deteriorates. I imagine the emotional blackmail - in desperation as much as anything - will be immense when that time comes.
In fact, even if that scenario were to play out and your DSis were left to continue her existence until her natural death from old age, depending on her core organs' physical abilities (i.e. the basic 'staying alive' ones), as she's nearly a decade younger, she could well outlive you.
Do follow the caring advice given in this thread - I truly hope you can all find the best kind of closure on this before long 