Sarah, good luck to you and your sister.
If you stick to this 100% you could lose that 3 stone by your holiday! I started at over 18 stone and found the loss bigger at the start.
As others have said, take it one day at a time in the first week or 2. It does get easier, honestly, I'm on my 19th week and I find it a breeze! I cook and bake with my 3 children and do the weekly shop with no problems. I do find myself tempted by fresh salad and salmon and prawns but that is such an improvement on the cr*p I used to shovel in my mouth!
You tastebuds will change so it is always worth going back to the flavours you didn't like later on. I started on just strawberry then introduced flapjacks (not the tastiest but I like them and enjoy being able to chew occasionally, many don't like them!) About 6 weeks ago I tried a vanilla shake with coffee as a cold latte and loved it! Now I have one every morning. 3 weeks ago I tried the chocolate and liked late so I alternate between strawberry and chocolate for tea. I hated the soup in my first week and haven't yet had the courage to try it again but may do as the weather gets colder. My problem, I think, was it was always too lumpy and salty!
Try the chocolate made with mint tea for a choc mint shake, I haven't tried it but others say it is yummy.
Try adding different amounts of water till you find an amount that suits you. I make my shakes with 400mls of water and fine them lovely - I prefer the strawberry one to those in MacD's!!!!
Good luck and hang in there! You WILL make your target!
x