Of COURSE you cannot read her mind.
You need to do up HER astrological birthchart and look at it.
Find out the exact time she was born .. go to alabe.com or cafeastrology.com and enter her time, full birthdate, and place of birth.
Her "Sign" is ONLY her SUN.
In astrology there are actually 10 "planets" with each planet representing a different part of the personality.
Sun is her need to like herself .. so Sun will NOT give you any clues for her emotional nature. MOON is the planet that indicates someone's emotional nature, and whatever "aspects" Moon makes to any of the other planets tells you how easy or difficult it is for her to work with her own emotions.
"Aspects" are specific distances, and shown by blue or green or red lines connection one planet to the other . .indicating interactions between those two inner needs/planets. So YOU want to look at any red lines to her Moon, indicating inner conflict between some need(s) and her emotional nature.
Also, look for "red lines" to her Saturn .. since Saturn is how we handle reality, but when Saturn is "aspected stressfully" the person often has feelings of inadequacy and sees offense even where none is intended. My own mother had Venus square Saturn and was like this. She never outgrew it, which is a shame, since one CAN outgrow the influence of their astrological birthchart if they work on it.
Why would you do look at your MIL's chart?
Because we cannot change anyone except ourselves.
And most of what determines how WE feel about something is what WE tell ourselves about it.
So you can be less stressed if you understand that her emotions are her problems, and then resolve not to make them your problems as well. Also less stressed by feeling compassion for her own struggles rather than focusing on your own frustrations with her ... astrology can give you insight into her struggles, and that helps you feel more compassion and also make the whole situation for you less stressful.
But, yes, it can be very difficult to deal with someone who gets offended at the drop of a hat. I hope something here helps.