Don't have to be drunk for alcohol to impair judgement. Most agree impairment starts with the first drink and gets worse with each subsequent drink consumed.
SS usually doesn't interfere with those they guard, even if what they're doing is illegal. (See: Friday, June 1, 2001, Bush daughters cited on alcohol laws - Police accused Barbara Bush of being a minor in possession of alcohol and Jenna Bush of misrepresenting her age for trying to use false identification to buy alcohol, according to a statement from the Austin Police Department.)
Presumably, their SS detail was with them in June, 2001, when they A) bought alcohol and B) presented a false ID, yet did nothing to stop the illegal activity.
Did the twins or their friends have guns?