Hypothetically, of course! Get it off your chest!
This is a judgement free zone.

I’d tell my SIL: - You have little to no self awareness. - You need to stop acting like you’re a victim in the problems that YOU have created. - You need to improve your conflict resolution skills, like, YESTERDAY. - Literally every family member (including your own family) can NOT stand you because of your lack of healthy conflict resolution and constant paranoia that the world is against you. - You’re not the center of the universe (SURPRISING, isn’t it??) - You’re a divisive, insecure covert narcissist that everyone can see right through. - Your forced and over exaggerated “nice girl” act is so obvious and OLD. - You are 100% the reason that the brotherhood between our husbands is ruined, thanks to your out of control fear of abandonment / jealousy issues. - You need to stop projecting said abandonment wound onto your children. The tighter you try to have a grip on / control them, the more it will create a self fulfilling prophecy where your children will feel smothered and abandon/hate you when they’re adults. I’ve seen it happen many times in other families and you are not exempt to the laws of nature. - Your fear that I will try to turn your daughters against you (like, WTF???) and not allowing them to speak to me (unless first asking you for permission so you can hear every word exchanged to ensure I don’t try to turn them against you) is borderline psychotic. - You keeping your daughters from speaking to me is only making them want to speak to me more. I’m seeing that right before my own eyes. They are losing respect for your authority and will not be controllable their whole lives. - You’re ruining your kids’ childhood by painting me and my husband, people who genuinely love and care for your kids, as “dangerous” simply because we can see through your BS and you don’t like that. - You are setting yourself up for more pain by avoiding taking responsibility for all of the above. - You need therapy, BADLY.