Title of Brother/Brother-in-law?

[Lisa in California]

We recently received a copy of a letter that was written in 1915.  The letter was written in New York and mailed to California.  The sender (Jane) would have been about 75 years old and the person receiving the letter (Patrick) was about the same age.  The individuals were related by marriage - Jane was the sister-in-law of Patrick (both were married to their respective spouses c1870).  I believe while they exchanged at least yearly Christmas letters, I do not believe they ever saw each other face-to-face after c1875.

Jane starts the letter with "My Dear Brother" and ends it with "I remain your loving Sister & Aunt Jane" (apparently sending the letter to her nieces as well).  My question:  given the facts stated above, would it have been common practice to call one's brother-in-law brother?  I'm guessing it could have been a term of endearment?  Or, is there a chance that Jane could have been Patrick's true sister?  Thank you in advance for any advice that you are able to offer.  Lisa