How much notice, if any, is required to notify spouse of intent to subpoena a spouse’s relative? How much notice does the witness require? Is there any way to prevent spouse and relative from communicating directly or indirectly concerning the case? This could be a case of a hostile witness, since the witness would rather not testify.
There is nothing specific about the time frame for informing a party of the other witnesses you intend to call, but when you subpoena someone, you do have to provide a copy of the subpoena to the other party.
You cannot stop people from talking, but you can ask the person testifying who they spoke to and what about when it relates to the preparation of testimony. Spousal communications are privileged.