Pal not confiding in me

[PotteryLottery]

I learnt from social media that a pal's sibling is recovering from cancer.

Pal hasn't mentioned this to me although it could have easily been mentioned in the context of something else.

I have confided lots of personal stuff to pal.

Should I say I was sorry to hear about her sibling, or just respect she doesn't want to talk to me about it?

Perhaps relative asked them not to say anything?

It may be that your friends sister asked her not to discuss it with anyone whilst she was going through treatment.
That is not an uncommon request.
Of course say you're sorry to hear about it to her.

I know a few people who didn't want people to know they were having cancer treatment until the prognosis was a more positive outcome. I have deliberately not said anything until they wanted to.