Going against the grain here to say YABU, on a few levels.
I've been in a similar situation being given £2000 while a sibling received ten times that amount, won't bore you with the details.
I think it's telling that this sibling is the only one who isn't a homeowner, and hasn't formed a steady relationship. I don't know if those two facts are linked in her situation, but it's far, far easier to get a mortgage when there are two of you to contribute a deposit, pay the mortgage, contribute to upkeep, and shoulder problems like redundancy that may befall any of us. If the rest of you have got partners and bought homes then it makes sense why if your parents wanted to give the property to one of you they'd choose her. A way of helping her to get on equal footing with the rest of you despite not having had the fortune to meet a partner like the rest of you have.
Secondly, I do think YABU to feel as though you have any right or claim over someone else's property. It comes across a bit like you feel as though it should have been yours or partly yours and now you're upset because it's been taken away from you. It was never yours, you and your parents are separate people and it's their home to do what they want with. Even if their choices don't make sense to you (or don't benefit you as much as you'd like them to).
'The rest of us work far harder to end up with a lot less' is a really horrible attitude to have. I worked hard to lift myself out of poverty and become a homeowner but that doesn't mean that outcome didn't also materialise due to a lot of privilege too. Meeting a partner is a major thing that not everyone is lucky enough to do. Being a homeowner (including with a mortgage, even if you don't own outright) is absolutely huge and something many people dream of but will never have. Your sister has I assume been paying rent for years while you pay towards buying your property, that doesn't make either of you more or less hard workers (you said yourself she has a career!).
What's fair isn't necessarily giving the precise same thing to every single child, and your parents have made their decision because they want your sister to have the home, not any of the rest of you. And that's down to them. If you allow something like this to ruin your relationship with them then I think you'll really regret it, there's no real way you can approach this with them without it seeming preposterous and entitled (imagine a grown adult crying to their parents because of what they've decided to do with a house they own, while being a homeowner themselves!).
I know you're coming across really bitter and quite nasty but I hope that's the shock talking and that once you've calmed down and processed it a bit you'll realise how unreasonable you're being here. I would hate to see it destroy your family relationships, but you do what you gotta do.