Sounds fabulous. The only things is get her to do are to make sure she leaves you with photocopies and digital copies of her passport, ghic, insurance documents and ticket(s) and maybe any credit card/etc too, so that if they do encounter a problem you will be able to provide some support. Likewise if you have a copy of her travelling companions' details and they have their parents having your dd's details. And she needs to take a photocopy of her details with her and have them accessible online (eg in a Gmail account) so that she can access them too if needed - if she does have a problem and loses her passport it's much easier to get it reissued and sorted out if she has all the info with her.
If also get her to have a money belt and/or money scrunchie/etc that she can put some money and cards in - she's not going to want to rely on just keeping things safe in her main bag and day bag!
Also make sure she knows what to do if she does have a problem - something is lost or stolen, will she need to report it to the police, get a crime number, report from the hostel or anything else? If she wants to claim on her health insurance, will she need to use her ghic first or does she need to call the insurance 24 hour helpline before doing anything for a claim to be valid? And so on... Different situations, different policies, different countries etc - just so long as she is aware of all the'obligations' that some of these things come with - if she were to have her bag stolen it would be gutting to discover that she wasn't covered because she didn't get a crime number at the right time or that her health cover didn't cover the initial treatment because she hadn't phoned up before going to the hospital...
Also try to persuade her to take pictures of lots of the little details as she goes around - tickets, name of places, attractions, stations, snacks, being in supermarkets or food shops, places where she stays, as well as the main sights and her friends - it will really help her remember her trip when she comes back and make for a much better photo book if she decides to go down that route for making her memories afterwards, bit will also be nice to talk through and show you and her sister on her return, without putting pressure on her at the time. If she is able to upload it to something like Instagram or Amazon prime photo storage (if you have Prime? It's got good free photo storage!) as she goes, and you're able to see what's going on then that's a bonus for you :)
Make sure she leaves plenty of space in her rucksacks and bag before she goes on case she wants to buy any souvenirs or gifts, and that she takes at least 1 plastic wallet that she can seal watertight to put all her leaflets, brochures, tickets, maps, guides etc in as she travels around.
Encourage her to have a fantastic time and say that you and dd2 are looking forward to hearing all about it on her return. Can she do anything to get dd2 interested or involved rather than worried? Even if it's just too take pictures of dogs that she thinks she will find cute or the biggest cream cakes she spots in each destination... Something that will change the narrative around and not make it as worrying...