I am reading Sense and Sensiblity for the second time in 8 years. I noticed that in the beginning when we are learning of John Dashwood's character, finances and his relationship to Elinor, Marianne and Margaret, JA refers to their mother has his mother in law. Really, she would be his step mother. A few paragraph's later she does call the elder Mrs. Dashwood, Fanny's mother in law. I was a little confused but then I figured the term must have had a slightly different meaning then? Is this true?

Thanks.