Sei who normally doesn't drink gets angry and starts drinking a lot of wine. The speaker is a maid and she thinks Sei is happy instead and says that.
I would have used 飲む. I know お酒に飲まれる (this), but it doesn't seem to be a mistake.
The subject and the agent should be the same: 聖が(聖に)お酒を飲まれるなんて. Maybe, it's something like "You let yourself drink alcohol", but it's not causative. I've never seen this usage before.
Please, make similar examples.