Children’s Book: 2 siblings (might have been animal characters instead of human) go on a train to visit family member in a swamp, meet witch & find a pearl

[Nina]

(Read this in elementary school sometime in the early 2000s)
(Also remember there being lots of pictures but I don’t think it was a picture book)
(Possible spoilers ahead)
There were 2 siblings -maybe a boy and a girl- who were illustrated either as human or maybe animals that had human qualities? Not sure. So they went on this train to visit a family member -maybe an aunt- who lived in a swamp or forest of some kind. One is the siblings meets this witch or creepy lady who lives there and finds -or is given? Or steals?- a pearl or rock or something valuable. Don’t really remember the middle part but the kid at one point drops the pearl into the water from a little bridge and it’s missing and they leave on the train without finding it -I think- and I don’t really remember the end but I sort of remember maybe the witch meets them somewhere after and gives the kid the pearl?
I’ve been looking and asking librarians for years to find this book pleaser help!

[Nina]

Still hoping to find this book if anyone has any ideas!