Book Sleuth discussion

Since I started this group to find a book, I will post first. I am trying to find a book that I read in the 80s about two sisters who are on vacation at the beach (not sure if it's West or East coast). They are in the US, I think one sister is in a wheelchair and there is something about them finding crabs and their mom plays a prominent part in the story. It might have been a Scholastic book but not quite sure. Please help! Thanks.