that is a hard spot, i have a young daughter also. i was an alcoholic for a while. so i know some of the signs of drinking and i watch her closely. so far she has no interest in drinking.
well one good thing is you know for a fact she is drinking behind your back. the question you might have to prepare your self to ask, what else does she do? common for kids to take prescription medication at there parties. you name it, if they think they can get a high, they might try it.
i dont know what to recommend for offending you. but maybe try a family intervention with kids and all. i think your younger ones are old enough and seen enough to understand what is going on. try taking her to a AA meeting near you with you and your husband. you can also think about trying a family therapist to help open doors of communication that you cannot open with her.
i hope this helps you a bit. but i hope things go positive for you.