How big the risk is is again subjective.
The risk of a baby dying is very small, as a proportion of the population. The numbers of those baby deaths being declared as SIDS related on the post-mortum is even smaller. Then within that small number within the small number, the cause of the SIDS death being prone sleeping will be an even smaller number
But the risk we are talking about here is the death of a baby.
Not just that, but the avoidable death of a baby because the parent chose to ignore advice. So really the responsibility if the worse happens sits with the parent. This risk might be small, it comes down to how your DB and DSIL will feel if their baby die because of something they did.
But all risks must be balanced. There will be risks due to exhaustion and sleep deprivation. There will be risks to a child who cries excessively. There will be risks linked to it her aspects of SIDS and things like accidental suffocation (which isn't SIDS but is another cause of death in babies).
There are loads of risks to balance.
Prone sleeping (sleeping on front) and baby sleeping on you while you are asleep are both significant factors in SIDS risk. But SIDS deaths in themselves are low. There is always a "This is a risk.... But balanced against this...".
Parents should make that decision. But it should be an informed decision. If you think they are genuinely ignorant of the risks then it's right that you (gently) offer leaflets or articles so they can make and informed decision based on facts.