Goodness, typical mumsnetters getting so judgmental when we don't know all the facts.
We don't know that OP never checked on the girls at all that night. We don't know if she knew that was the exact point the teenagers were to be drinking or if she had told them to stay in the girls room.
Those also saying she has sole responsibility and the other parents agreed she was the ones looking after the girls, again we don't know that. If my children go to a sleepover I am usually aware there's a father in the home too, who may also take some responsibility for supervising children. He may only be the step dad but he is still OPs DH and by the sounds of it they share another child together. So I don't think it's wrong to expect a step father to take some responsibility.
OP has said she regrets the decision to allow any alcohol but by no means was it ever meant to be a situation where anyone would be drunk. It could also have been a case of they were supposed to be having the drink at a different time in that evening and stepfather has given them it when OP was elsewhere. It does not mean she said yes off you go on your own at this moment and get drunk.
I think OPs mistake was posting vaguely on here to be judged and blamed for something her DH has done. He is also supposed to be a responsible adult and I'm sure OP isn't the first person to trust their own husband to be a responsible adult.
OP hasn't supplied the teenager with a large amount of alcohol. She and the other teenagers parents agreed on a small amount, which for all we know could have been supposed to have been drunk at a different point the evening, with a meal or anything like that.
OP - your DH was definitely out of order and I hope you would never trust his judgement again. Even if he meant no harm, he's put himself in a really bad position here and made himself look like a creepy stepdad. I hope your DD and her friend are both okay and I hope this doesn't cause any major issues.