I think it was your SIL"s choice and even though it's something you wouldnt' do, she decided in that circumstance it was ok. I wouldn't think she was doing it to be irresponsible, I think she just made a choice for that moment. It's easy to say the "what if's" but really, it' s their decision.
However, if you really feel you want to say something, take it away from them and make it about someone else. ie "I was a bit concerned about you holding the baby because there are some CRAZY drivers out there, especially at that time, coming home from an arvo/night at the pub etc", then she won't feel it's personal. xoxo