Benefit fraud is a crime with a maximum penalty, for the most serious offenders, of ten years in prison. From what you've described from your SIL's case, I'd be surprised if she met the custody threshold, but it would still be treated seriously by the Courts if it ever went that far.
This isn't a victimless crime - everyone who pays tax are victims of benefits cheats.
In your SIL's case, the reason she is committing fraud isn't even to put food on the table, it's so she can increase her savings.
I don't share the view that benefits claimants shouldn't be 'allowed' to save but I do believe that benefits should be a safety net for those most in need and not a lifestyle choice.
PPs have suggested fraud is okay, because others (politicians) do it. This is clearly nonsense, you can't say a crime is acceptable simply because some people get away with it. They wouldn't be saying this is it was, say, a sexual assault!
You've raised your concerns with her direct and by the sound of it she has chosen to continuesher criminal behaviour. It's a tough decision - but personally I think you should report her to the benefit fraud hotline.