To be honest if her kids are happy doing school work for that many hours a day, then good for them. I don't agree with the posters trying to shame her for working her kids through the holidays. With enough prep, the right mindset, and kids used to it then school work can be fun.
The idea that school work is inherently bad and boring is dangerous. You get kids trying to flee from any kind of work. We don't need to push the mindset that this woman is doing something bad so that you don't feel bad about what you're doing.
That being said, you don't need to feel bad about what you're doing.
You and her, and yours and her kids are different. You're going to approach schoolwork and life in a different way.
Knowledge is a good cure for feeling insecure. Check out their curriculum and what the end goal of their courses are (topic areas they'll need to learn, exams they might be learning toward, written assignments.)
You're working yourself, so your goal isn't to learn it all alongside them. It's to make sure you (and more importantly they) have an idea of what the big picture of each subject is. So they know where they're going and that they're learning each subject enough to be able to do whatever work will end up deciding their grades.
That way they'll also be able to notice any problem areas. There are so many lovely revision books that will help them plug any gaps.
And for study skills themselves my favourite is 'how to become an A star student' by Cal Newport. It's great because it's not about the usual grind through boring schoolwork.
It's about how to study efficiently and learn things quickly and easily as possible to make sure you've got it, and that you're not wasting time not really learning. It also covers essays and is applicable to multiple topics. It's aimed at college students but is a fun easy read and is so much about the fundamentals of learning that it'll be useful for anyone aged 12 or over. I'd recommend it even if you don't think your kids have problems studying.
To be short. The focus of the above is just about making sure they're able to assess how they're doing and plan their learning. You just need to learn enough to be able to ask them enough questions to know how they're doing and for them to feel they're able to tell you if they're finding a topic difficult.
And just for your browsing, take a look at the forum 'well trained mind.' It's a massive community of home schoolers and after schoolers. It's American based, so if you do decide to go in a level deeper and look at curriculum you would need to play about with the levels, etc. I'm recommending it to you solely so you can look at some of the posts and be reassured that people do this kind of thing all the time. And most started knowing zilch about it.
Plus the forum can be a bit addictive (but that may just be me).
So she's doing well and it appears you're doing well as well. Just try to get a grasp of what they' need to cover so you can reassure yourself of that. Don't be too shy to send their teachers an email to ask how they're doing and if there are any weak areas they need to concentrate on. If you and your kids and your teachers all manage to work together it makes it so much easier and will hopefully help you feel better about what you're doing.