How to respond, In-law sent perplexing email!

My SIL just forwarded me an email that was a response to a previous email my S/O sent her. She also asked via Facebook for my email addr, which I know she has. (what…?)
From her email I gather she is royally pi$$ed at my S/O, but also from the scant details she included I can’t figure out what has caused the ruckus. (Some bother about “he took something from me from our past- blah, blah, blah”.) Doesn’t sound like anything of value and I haven’t seen confirmation of the ‘item of contention’ in our abode.
Yikes, have never dealt with this sort of family ‘dynamics’ before…..Gut instinct is to tell her to pull up her big girl panties and deal with it -like the 50+ woman she is (as am I) and to stop throwing dirt around.
She has ‘banned’ my S/O from contacting her 25 & 27 year old “children” as she states ‘this causes them unbelievable stress’ – yet they converse with my S/O on Facebook about the latest NFL standings, etc? I am thinking able-bodied adult children can establish their own healthy boundaries.
Totally not liking this, but recognizing since she’s an in-law I have to eventually address this…I feel awkward not addressing her Facebook post because I am actively posting with other ‘in-laws’ there.
Help?
The fun of being a jelly, living in a fishbowl