Tesco have great development programmes. She needs to arrange to speak to her personnel manager and ask about their 'Options' programme. She could be sent to workshops that would boost her confidence and soft skills.
Its not her ideal role, but it will show continued personal development and make Tesco a little more interesting while she looks for something else. She could then maybe transfer to a company like Pets At Home where she could use her degree.
This covers what I was thinking. The job she has right now is an opportunity; it will take her somewhere even that's only demonstrating her commitment to a job, obtaining great references, developing all her transferable skills, and providing an income which can potentially be used to support widening her horizons in her free time.
Along with telling her sister to wind her neck in. None of us has a time machine, so "we told you so" is unhelpful and unkind.
Having a plan, or even just the hint of a plan, in the back of your mind can make it a lot easier to deal with a job that isn't what you'd like. Maybe to start with that plan could be "I'm going to use tesco to make myself more marketable and employable when I'm ready to move on".
The huge positive here is that she has a full time job with a company that will invest in her if she pursues it. Don't write off the significant of that. If she can show herself to be determined, hardworking, motivated, and adept at learning new things the then other employers will be more interested (which is what I mean by being marketable).
The other angle here is that there seems to be an undue amount of focus on whether or not she's in a perfect/dream/successful job.
What about making her day to day life meaningful and enjoyable to her? Aside from all the stuf about skills, the job she has now offers financial security and stability. It means she has options to build up other parts of her life.
If she can have more going on for herself other than "I have to get a better job, I have to build my skills, I have to get better at interviews" it will most likely help with her mood, her sleeping, and generally how difficult this all feels for her. Hobbies, activities, socialising, relaxing activities that are not job or employment skills related are very necessary here!
The job she has is just one small part of her life and who she is. It shouldn't be the central or only part of it. I write this as someone who has been there with not wanting to go to sleep because I'm dreading another day arriving. She is not alone with that experience. How she feels is valid, and dismissing it won't help, but there's loads of ways to improve it.