Well I would assume the usual "don't do anything illegal" in the house clause in the contract would cover it. They probably made it clear that if she drinks she will be cut immediately just like she would if she assaulted someone or pulled out a crack pipe from her luggage and smoked it.It isn't illegal for there to be minors in a house where there is alcohol. It is illegal to allow them to drink, so as long as they do things to make it clear they are not allowing or requiring her to drink as part of a challenge, or the show in general, it shouldn't be a problem for someone under 21 and a bottle of beer or a glass of wine to be in the same room.
I'd guess they usually require the 21 because it is better if more hamsters get drunk when alcohol is provided (that they do not in any way require or force them to drink). More drunks equal more drama.
I really don't like the drunk drama, so if this means no alcohol in the house I'd be just fine with that.

Summertribe and the living is easy!
maybe they could just pass around a pipe instead..oh wait, pot's illegal...dammit!She's two months shy of 21....not sure how long the show goes. It would have been great to see the birthday party, wonder what her Dad would give her? A homemade tatt?
I like em drunk...just wish they'd stop ingnoring the over 35 set...us 40 somethings like to see our peers make asses of themselves too!