My mum has this, she was diagnosed finally 3 years ago. Looking back she has been misdiagnosed for years.
She is now on Cellcept, Hydrocolorquinine, steroids,and permanent Fentanyl patches (morphine for pain relief). Steroids are only meant to be a temporary measure but at the moment they can't get her off them without her having a pain crisis, so they are at a maintenance dose just now.
She also suffers from extreme exhaustion on occasions, and would be incapable of doing anything but sleep. She has been left with heart and lung damage as a result of it too.
Mum knows of other people though for whom the condition has been more easily managed, so she is just unlucky. She is getting a bit better though, and went 9 mths without a crisis which is the longest she has gone since diagnoses.
It's a horrible condition, with loads of symptoms, tthe Lupus UK site is great for info and support, mum uses it a lot.
You may find that your brother may have to take carers leave on occasion to look after her or the children.
Is your SIL under the care of a rhuematologist? at a specialist centre? Does she have a Lupus nurse? Mum's has been a godsend in terms of info and support and help. It's not very well understood really in the wider medical proffession, so your SIL will pribably have to educate herself then her GP.
I just realised what a glum picture I have painted, but it's not all bad, just our experience. There are people oiut there who find that with adjustments life goes on pretty much as normal. I hope your sil is one of them!