My wife and I have a joint checking account. We added our married daughter's to the account in case something happens to us. One of our daughther's owed money and the bank she owed the money to garnished our checking account. Is this legal?

They may freeze the account, You can file in court to prove that all of the money in your account came from you or wife, and is not her money,
But this will need to be done quickly.

If she was added as joint owner of the account, they can "assume" it to be her money till you prove other wise