Update
Several of you expressed the opinion that I put a bug in mom's ear about some of this stuff. At the time I didn't do so, only because I don't KNOW mom; and these folks live in another state entirely.
This week things have become worse, though. Stepmom has started cutting and pasting entire e-mails between mom and dad. She wasn't cc'd on the e-mails, but apparently her DH lets her read them. She is not deleting mom's name or e-mail address, so now it's out there.
The allegations and slander have been getting worse and worse. Yesterday, she posted that her DH had lied to mom about whether he'd be home for an overnight last weekend; stayed there for drop off and then left on a camping trip. SM had them over night. She admitted that because her SK's were "miserable lying beasts" and wouldn't "confess" to whether or not they were spying on her for mom, they had to go to there rooms without food for 24 hours. I was shocked, but e-mailed her saying "surely you must be exaggerating", but she replied that she absolutely had NOT been, and didn't see how it was any worse than the fasting her parents had HER do as a girl for religious reasons.
Am I meddling or overstepping to anonymously let mom know now? I'd like to do so. If not, I will call CPS. Just trying to find out whether hearing it via Facebook and e-mail is too much "hearsay" (I did save a screenshot) for CPS, or if this is simply a different, poor, but acceptable variation in parenting methods.