Another vote for sainsbos, their free from range is brilliant, as it their meat-free range. Asda's are awful, their chicken pieces are vile. Morrisons and Tescos meat-free ranges are good too.
The people I live with are vegan and gluten free, they have amazing looking meals most nights consisting mainly of beans/rice/gluten free pasta and a big variety of veg with each meal along with some vegan sausages. They also throw seed mixes on meals, vegan 'parmesan' and nori flakes for added vitamins and taste. You can get loads of little things like that (powdered superfoods are handy too!)
She can bake treats for herself using coconut flour (can be found in independant health food stores or online) which is brilliant, or quinoa/millet/spelt/hemp/teff flours which are all easy to use once you get the hang of it. There is a good brand of gluten free flour mixes called Dove's Farm which waitrose sell and independant health food stores too.
Tofu is extremely versatile once you get used to cooking with that too. And dried soya is handy :) Currys are great too, you can make curries out of any veg and add quorn if she choses.
Quorn has got dairy in it, minimal amounts but it is there so be wary of it, soya, tofu and seitan may be better alternatives.
I used to be the same as your sister, I am a veggie who had big food issues (mainly regarding textures so had a lot of trouble with vegetables!) as well as a slight intolerance to dairy, so I can't have meals with a huge amount of egg/milk in and I have only just managed to start trying new things and improving my diet. I was very frustrated with my diet before this year! I still struggle with fruits but eat lots of freeze dried fruits which I bought from healthy supplies (online). They're somewhat expensive but very tasty and handy as a snack.
If she's after recipe books to try, me and my boyfriend bought the Quorn Cook Book in September and I have never used a recipe book as much as I have that one, it's superb and half of the recipes can be adapted for use without quorn too! I use it at least twice a week for recipes.
I've started reading a few recipe books on raw food too and it seems very inventive, maybe she could look into that for inspiration too. there is a good raw chocolate cook book that's trendy looking and has amazing recipes in!
But get her to start small. The reason I struggled for so long was because I had no clue where to start. Once she's started she'll start experiemnting and feeling comfortable and her diet will naturally evolve. Also, she has to really want to change her diet to put in the effort!