I am trying to remember a book series I read but couldn't finish in middle school
There were two siblings, a brother and sister, they go to live with a relative for the summer. Things are weird. Mildly sus. SOMETHING happens and the kids drink this magic milk(?) And can see the magic world. Faeries, a giant cow, a golum etc. The dangerous pond turns out to have nixies, or sirens I don't remember. The family maid turns out to be one of those nixies or sirens and rejoins her sisters until they trick her back onto land. The sister becomes a fairy queen, I think? And can see the magic world without needing to drink the magic stuff. The brother accidentally turned a faerie evil by trapping her in a jar overnight. He is almost killed over this. The brother also makes a deal, I think, and is eaten by a giant frog and saves himself by going inside a giant pea pod and then eats his way out of the pea pod when the frog(toad?) Turns to stone. I never finished the series because the middle school didn't have the full series and now I'm going insane trying to remember it.