I'm sorry for your trouble, OP.
Life is so difficult these days that money, and the love of money, means much more than it should do, or would do, in a fairer society. Maybe your SIL is struggling to get by. It's not necessarily hurtful or wrong to think something along those lines. It would flicker through anyone's mind. The problem is how it was put and that, I think you should address, otherwise you might end up drawing away for no good reason.
Without knowing your SIL and how she thinks and talks, I don't think anyone can tell if she was being callously speculative, or just speculative. It was disrespectful and it's disappointing to think that people might say things like this behind closed doors, of course. But she wasn't speaking to someone who was vulnerable or grieving. She could just be very blunt in a heart-on-your-sleeve kind of way. I can think of people like this. Or she could be horrid.
Maybe you could consider just going to her , or writing a card, and saying, 'I know the text wasn't meant for me, or meant in a horrible way, but I've ended up knowing about it and as I'm feeling fragile, it ended up being upsetting. I know you wouldn't have meant it that way but I needed to tell you because I don't want to run the risk of it driving a wedge between us.'