I'd go with something that didn't sound too judgemental, in case she did make the comment. Maybe "Sis, I noticed a comment on FB in reply to Friend's post that didn't feel like your usual writing style. I've heard of people getting their accounts hacked, so I wanted to give you a heads up. If it was your post, please ignore this."
That's wordy, so I'm sure someone else will say it better...but if you attribute it to "not your usual style" it would come across as you being actually concerned, and not you trying to call her out on an "off" comment.