Just my first impressions here, because you know your brother and sister-in-law and I don't. Does your sister-in-law know that your brother has told you? And if not, would she be angry if you knew? If she wouldn't be angry, then you can mention to her how sorry you are to hear about her illness, then take it from there.  And don't ask questions. If she wants to talk about it she will. But try not to worry too much about saying the wrong thing. There is always a risk of that in these situations, but it important that she knows that you care, even if you are not particularly close. Good luck. Harry.

I can understand your worries as - forgive me - your family don't sound like cosy communicators and you want so much to be helpful and do the right thing.  You have told your brother you are coming so he must know that your sister-in-law's cancer is going to be the main topic of discussion.  I think the important thing is to let your brother and sister-in-law know you want to be there for them without asking too many probing questions.  I am someone who always asks questions but usually preface it with "I understand it is not primarily my business but I want to be helpful if I can" although perhaps not in such formal words.  Then play it by ear.   You can't gauge everything correctly first time off but at least you will have let them know you want to do the right thing and be helpful.  Don't  know if this makes any sense - best of  luck and I am sorry you have  had this bad news in your family.