Instead of asking her any questions, which places the onus for polite conversation on the your SIL and which she may still be struggling to answer even privately for herself, offer open ended, undemanding statements.

‘Hey, I just saw this [whatever] and it reminded me of you.’

‘Morning, [name]. Just wanted to check in on you and let you know that I’m thinking of you. And no worries if you don’t want to text back just yet or even at all. But if you ever do want to talk or simply hang out, let me know. :)’

Let her know that with you, her grief is just as welcome as her happiness, and that you are glad to be someone for her to lean on.