Seems like this thread is going around in circles.
OP, you've said that you suspected your SIL was unsafe to drive, but then when she said she was fine, you say you trusted her judgement. Setting aside the issue of whether your SIL has a drinking problem or not, it seems like the main problem is that after choosing to believe that your SIL was okay to drive, you're now second-guessing your decision.
This is the conversation that needs to be had, uncomfortable as it may be. If SIL really was unsafe to drive, yes, she made a bad choice that needs to be addressed for the future, but so did you, by not being more insistent about her not driving. The safety of the children trumps the discomfort of a tough conversation.
So here's how it could go.
"SIL, I need to talk to you about something difficult so I hope you'll bear with me. The other day when you were feeling really dusty after your big night, I was worried about whether you might be too hungover to drive. You said you were fine and I took you at your word, and obviously you did all get home, but I can't help feeling that it was a mistake on my part not to be more insistent. It's hard to judge whether you're really okay or not when you're so out of sorts.
I'm not judging you, I just love you and your kids and I know I couldn't live with myself if something had happened. So I'm just letting you know that if a similar situation comes up, I'm going to insist on having the keys until I feel really sure you're okay to drive. I hope you'll see where I'm coming from. I'd do the same for anyone I love."
A direct and loving conversation is your first action. This is family. You don't start ringing police until you've put in some effort yourself first.