Thank you for your response Splendra!!!
Bullet dodged!!
I did call SIL today and left a rambling message. She ended up coming into my work, and told me that 1) she is planning to renovate that back place and 2) she DOES have someone in mind (the prior tenant's mother who has had her eye on it for years)
So, I was able to truthfully tell coworker it was not free. I did fell like I kicked a puppy though- she was pretty sad as she had already been making plans!! Also, she was asking me about SIL where she worked, etc. I mentioned X trucking company. CW then asked if I could see if SIL(who she has never met) Could find her a job there!!! AARRGGHH!!
I do feel better though. Need to learn to keep the mouth shut.