I can’t remember the name of a book I read when I was little

This is a kinda dumb and not related to periods at all but I read this book when I was younger and I cannot figure out what it’s called. It’s a kids book and it had a lot of illustrations in it. It was about this brother and sister (I wanna day they were mice or something but I could be wrong) and they go on this train to visit their aunt or something who lives in a swamp. So the aunt is a witch or something and the kids have either a pearl or a marble or something that the aunt wants but they drop it off a sock into the water. I don’t remember what happens but they take the train back and I think it ends with the kids wearing fishnets and the aunt is like watching them from behind some boxes. It was really creepy but I need to know what it’s called... help?