How long ago did this happen? You've already talked to the police, they've looked at the nanny cam tape and so forth...are they in the process of deciding what to charge you with?
And for what it's worth, I can easily see how it could have been worse. "Worse" would have been actual interaction by you or your lady friend with the baby. "Worse" would have been some visual acknowledgement on the tape that you knew the baby was in the room, and you fooled around anyway. "Better," on the other hand, would have been taking the lady in there, noticing the baby (if only by happenstance), and immediately leaving.
When you say you threw the blanket over the crib, was it the crib railing, rather than into the crib? If over the railing, there was no danger of the baby suffocating.
It's still an interesting question why you picked the baby's room to sneak into, even if you didn't think the baby was there, rather than the room you'd been given to sleep in.
I agree with everyone else that you almost certainly should get a lawyer. It seems much too risky not to. If by some miracle the case doesn't go very far, you probably won't owe the lawyer all that much money (as lawyers go). But if the case is pursued, you absolutely will need a lawyer. But if you hire someone now, rather than waiting, they will have more time to investigate and prepare, giving you the best chance. Err on the side of prudence.
I must say, if this was me and my baby son and my brother, I think I'd have a hard time turning this over to the police, since there was ironclad proof (via the nanny cam) of what did and did not happen. I would yell profusely at my brother, tell him to get out of the house, and probably have nothing to do with him for a good long time, on the grounds of disgust and inappropriateness (bringing strange women from bars into my house? Seriously?!??), but I'd have a hard time turning my own brother over to the police and potentially ruining his life. From that perspective, good luck to you.
[Edited to add] Can I assume you've tried to REALLY REALLY apologize to your sister? Maybe that's too simplistic or naive a question, but the best outcome for you would obviously be if you could work it out with her privately.