YorkshireQueen, you asked AIBU, and we gave you our answers, which you obviously don't agree with, and you aren't prepared to change the way you think, or to change your attitude towards your eldest GD. That is your choice.
On paper, paying £80K deposit, on a £190K property, leaving £110K mortgage, the monthly repayments probably are doable with their joint income. However, once the bills start coming in, which your YGD has never had to pay or budget for in her whole life, it may be a very different matter. Not only with the monthly utility bills, but all the other outgoings that homeowners have to pay for ... Solicitor's fees, surveyor's fees, buildings insurance, house contents insurance, the costs of buying furnishings/carpets/curtains/etc., weekly food/household bills, occasional repair costs when something breaks (washing machine/tv/etc.), which can really put a dent in your monthly budget if you don't already have money put aside for these things. TV license, internet, mobile phone, clothing costs. Do either of them own a car? ... Petrol costs, services and MOT costs, repair bills, tax, insurance, etc. If they don't run a car, then there's still travel expenses to/from work everyday. Do they plan to ever go out/socialise at weekends for drinks/meals? Do they ever plan to go on any holidays?
I'd be worried that after the initial euphoria of buying their first home together, they may find themselves in financial difficulties, unless they have sat down and seriously (and honestly!), worked out all of their figures to include all of the above (and probably more - there's always more!).
Far from being excited for your YGD, I'd be telling her to be very cautious, and to make sure they know exactly what they are getting themselves into.
Maybe your older GD, being the sensible, grown up one, who pays her way, can see how this might all end up for her younger sister, who it seems has never had the responsibility of having to pay her way, or budget for anything. It seems she has simply flounced through life, saving all her salary, scrounging off both her parents and her sister, to enable her to save a little nest egg so she can now jointly buy a property with her boyfriend. Who, it seems, she will be scrounging off for the foreseeable future.