Would I be able to punish my 18 year old for smoking weed?

I have an 18 year old daughter and a 17 year old stepson. I caught them smoking weed with their friends in my stepson's car. I grounded him and took his car and phone since he's still underage but I was wondering if I would be able to successfully punish my 18 year old as well because I know people say you can't because they're adults. She's still under my roof though so..

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