Hope you don't mind a few questions, I want to make sure I understand what you are telling us.
Hi BabyDucks, I don’t mind at all. I’m being somewhat evasive because still live with fear H will uncover the posts. Ugh.
who did your sister do this not nice thing too? You? Your husband? Your family?
My sister and I were exchanging emails and trying to plan a family get together. She was not being very understanding that my family’s schedule was difficult and I couldn’t yet pin down an exact date of arrival. She scolded me and tried to insinuate that my H was controlling too much of the trip (wish I would have listened then!). But his job is demanding and sometimes things wrap up sooner/later than planned, so was too soon to be specific. I think I said something to the effect of that I’m sure she would consider her family’s situation when planning and that this was all I was doing. She threw out a previous issue with my H but in a kind of vague insinuating way. There was nothing there, she was just throwing a jab. H caught wind of it (long time ago and we had shared family email). He of course drew crazy conclusions to the comment. I explained the situation but he just created a story around it. I just took a step back from her thinking he would calm down and it would pass. Just as it was becoming a thing of the past, the next family incident occurred.
So Mutual Friend(s) got in trouble for hurting their own kids? someone else's kids? a one time event? a long series of abuse? the hurt was sexual? verbal? physical? an argument at the soccer match? something more serious? Your sister condemns the hurt? thinks it's no big deal? somewhere in between?
It was someone my parents brought into our home and raised from about ten and on. Nearly thirty years later this person was working with children and had child porn in their possession. None they took, no active abuse alleged on their part. Possession. Went away for it. Because my H is a L, my family wanted his help. He refused. I agreed. Children are deal breakers. My dad tried to suggest he confessed erroneously, etc. I don’t know if he actually ever told anyone else (only us because of H possibility to help). I don’t know if my dad was in denial, believed he didn’t (they had evidence and he confessed) or just didn’t think it was a big deal because he didn’t “actively” hurt anyone. (I disagree). It was a tough subject with dad and we chose to not continue discussing. Prior to this incident, I hadn’t seen this person in 3-5 years.
Anyway, my sister still engages with this person although they live far apart. It’s slimly possible she doesn’t know or that she isn’t aware of how bad it was. Idk. Anyway, our youngest were little when this happened and so it’s been kind of a lingering threat by H that my family is a threat.
do you think there is any reasonable current risk to your kids? do you feel your sister would be protective of your children in the typical fashion of a family member?
I stay away and there is no threat. I believe my sister would not directly/intentionally be unprotective, but if she doesn’t understand the magnitude or “threat” she may unwittingly add risk.
Now H has a few family members that are a bit less savory. Small time drug dealer (like pot to friends) and a fairly heavy drug user. We still see them in controlled situations but they aren’t child risks? And H’s brother was arrested for resisting a po at a traffic stop. His hot head got him in a lot of trouble. We still see him regularly. He’s a good guy (I suspect PD behind closed doors based on SIL stories she’s shared with me in confidence). So this may provide some leverage, which I would never normally do, but...

There is no way to control her social media posting. You can ask her not to post about your visit. and explain that you are trying to avoid conflict with your husband. what she posts or not is up to her. you can't manage your husband by controlling another person. better to spend time and energy coming up with a plan on how to respond to your husband when he learns you and your sister are in communication because eventually he will.
My hope was to keep it quiet until we can move about again. But, I can’t control it and ultimately I’d like him to know because I believe he will portray me as having no family support in court, which is true. It’s my own doing for not handling better, although not entirely sure what that would be.
don't share yet. start small. build rapport. allow her to express herself. speak in general terms. lay ground work. ask to speak again. don't put any pressure on your sister to understand or yourself to explain perfectly.
Thank you. This makes sense. Ideally I’d like to have my sister back but I destroyed the relationship so I have to accept it would only be her good will and likely none exists anymore. I will have to accept that.
So I plan to keep it short. Simply explain some risk/reason for keeping it between us for now. And then just apologize. Tell her we can talk again if she wants. Just leave it in her court.
Thanks BabyDucks.