I'd say your eldest GD has done quite a bit. She's paid towards the upkeep of the household, whilst your youngest GD has done nothing but save pretty much most of her salary for years. You say your eldest GD's contributions were by choice, so, basically, she was being a grown up and wanting to pay her own way, without sponging off her parents, kudos to her! Perhaps, for fairness, her parents should've accepted the money and put it to one side for her, so she'd have a nice lump sum to enable her to also buy a property. Not very fair of them to accept it from one daughter, and not expect anything from the youngest.
I'm not in the least bit surprised that she's fed up with listening to all the talk about her sister buying a property, and all the excitement and congratulations she's getting. People can be happy for your youngest GD, but this all sounds rather over the top. She hasn't even got a confirmed mortgage offer yet, let alone gone through the lengthy process of buying the flat, which doesn't always go smoothly. Probably a little premature for congratulations. I'd be sick of listening to it too.
So, you did ask AIBU? Yes, you are.