The Miranda warning only has to be given to a person being interrogated in custody, and on the premise that the wife is not in custody, the police do not need to read her the warning. Therefore, anything she says can be used against her, or somebody else, unless there is a separate reason why the statement could not be used.
The wife may invoke the spousal testimony privilege, in which case she cannot be compelled to testify against the husband. The officer could theoretically testify that the wife said "I washed blood out of his clothing", but this is an assertion made by an out-of-court declarant to prove the truth of the question at hand, i.e. hearsay. There are numerous exceptions to the definition so that in some cases, the statement would not be hearsay. If the wife refuses to testify, that cuts out half of the exceptions, but maybe the wife is a co-conspirator.