I feel torn. Torn between giving the OP an enormous hug (apologies if that offends), and wanted to chuck a brick through the computer screen at a few of the comments.
OP YANBU. Are you being "oversensitive" - well, maybe a tiny bit yes, but mostly a great big no. As you acknowledge, the fb posters probably didn't intend any offence. Some of them may not even have registered whose fb feed this was on, and just posted what they would post for any friend. Some will have seen who it was, but posted without thinking. And some...I suspect some may even have agonised over whether to post what they would always post, so as not to treat your dsis differently, or whether to put something different. And none of us can know who are which. I think you did really well, in your (completely bloody understandable) distress at the "lucky" comments to come on here, and vent your feelings anonymously. It would have been so, so easy for you to put something on fb, which might have "scared" people off posting anything in the future. But you didn't. You came here to let off steam. Sounds very very reasonable of you to me.
But why don't people think and remember, before they speak? Well...a bit of it is probably just the nature of the internet (do you know the one about thinking once before you speak, twice before you act, and three times before you post online? - I remain convinced that most of us don't think at all before posting on fb...). But part of it...someone above mentioned there being a timelimit after which those not directly involved seem to expect people to be "over it". I don't think that's entirely how people think...more that what is so naturally and devastatingly in front of you every day moves slowly and steadily towards the back of other people's minds, and it takes more and more to make them think about it. Which feels incredibly bitterly frustratingly wrong. And yet...we all do it to some extent. And is probably partly a survival mechanism - for people to be putting the mundane business of their own life way ahead of the recent traumas of their non-immediate-family was probably essential for the human race to survive.
But, yes, the term "lucky". It almost feels deliberately chosen as the worst possible word they could pick, if they held a hold bloody research project to find one! It isn't deliberate. You know it isn't. But you can't just switch off your normal emotional reaction.
I'm not surprised you're angry. In your position I would be fucking angry. I would be angry against fate, or god, or timing, or someone who caused the accident. God help me, I might even feel angry at my parents for not being there, or even my sister for not being able to support me. And then I would feel ridiculously guilty, then angry again... I have no idea where you are up to in your grief, but I do wonder if you have ever had the chance yourself to fully grieve, or if you had so much that you had to do, and be, as big sister, that you did that "locking it away" thing - that so many people have no choice but to do, but can make life feel even harder still, at the worst time possible. And yes, I'd probably be pretty angry at these people on fb. I'm just not sure that they'd be entirely the real target. (Or you may feel I'm talking bollocks - in which case I apologise for getting it wrong).
I don't think your sister is lucky for getting nice presents. I don't think either of you are lucky, to lose your parents. And, whatever may become possible to help your sister to express herself, of course she's not bloody lucky to have been crippled in this way. But from your postings I very strongly believe that she is lucky in just one, but very important, way: to have you as her sister. I have read your postings with tears in my eyes at the way in which your love for your sister, and your desire to help and protect her, shines through in every word. And I so hope for you both that one day she is able to communicate enough to express what I'm sure is in her heart - that you are amazing, and she loves you to bits.