I tried to quickly check up on the Sids and co-sleeping thing. As far as I could see in searching online through a few articles,

- the jury is out as to whether it really helps, because the data collection has been dodgy in the studies - there are too many variables, and not enough detail gathered when anyone has actually tried to study it. (maybe this has changed this the articles I read were written, though)

- one theory as to why it might help is that the baby sleeps more lightly when co-sleeping as compared to on their own, so they don't get into the deep sleep that can (apparently) result in breathing failures (I know nothing about the mechanism of this).

- another theory is that it has to do with the improved temperature regulation that sleeping by a larger body (the parent) provides.