It's hard to say what should be done with so little information.

If the girls generally get along, this isn't much different than how siblings normally treat one another. You fight, you say awful things, you make up, you fight, you say awful things...

A bigger issue is whether it was a private conversation or out there for the world to see. This is a huge point. If it was private, (and I hope it was) SD might have just been venting to a friend the way we all do and it's unfair to penalize her for something that was private as long as she treats your daughter respectfully. She's entitled to her feelings about anyone, as long as she's respectful. Making a big deal out of it could cause a bigger rift between the girls and a new rift with you, which would be a shame. Try to separate love for your daughter from this so you aren't overreacting because it sounds as if you and SD get along well.

If SD wrote nasty things for the world to see, that's a much bigger problem and I absolutely agree taking the electronics is a good approach. However, make sure you let her know the perils of publicly airing ugly remarks. Let her know people commit suicide over things others say online about them. Others could bully your daughter (and she's leading the charge for that bullying) and bad things happen when people get bullied. If none of that works, let her know that others might judge HER based on the ugly things she said about your daughter.

Ohhhhhh. I'm sorry she put it out there. These poor kids have no idea what power they have in their palms.

I think a good approach is to address the mistake of putting anything so personal and ugly out there - no matter who the subject is. It's fine if she's frustrated and wants to vent, but once it's out there, you can't get it back. Emphasize what it can do to HER reputation and why it's bad for her without even bringing up your daughter. Tell her you insist she take a break from electronics and social media for how long you decide to punish her because this incident made you realize she's too young to handle it.

As for the other issue...her relationship with your daughter, girls at that age can be so fickle. It might blow over on its own. You have to decide if their dynamic shows signs of something bigger. But if you think SD just hit a bad patch with your daughter that will mend itself in time, I'd skip addressing that aspect (unless she's disrespectful toward your daughter). I think there's risk in making this too much about your daughter. Everyone might retreat to their team and become defensive. Maybe by showing her you're not going to punish her for WHO she attacked, only for how she attacked, she won't hold the punishment against you and your daughter. You could say something like, "Your father and I hoped you'd find friendship with (daughter), but you have a right to feel the way you feel. If you want to talk about anything I'm here."

It would be better if your husband did the punishing with you right there by his side. Good luck.

13 is wayyyyy too young to have a social media account. Teenhood is dramatic enough without this unnecessarily added drama, not to mention the narcissism social media breeds.

DD should simply block SD from her social media then she will not see it.

I would start teaching my daughter the following :

Hon SD is jealous cause you are way better person then what she ever will be

Leave SD to do this, keep quiet and laugh when people talk about it, answer with, what ever makes her sleep at nigh ( people will soon see who's the mean girl and who's the pleasant one and they will reject SD more and more)

I will encourage DD to start ignoring SD, even at home, simply greet and be polite, but no friendly chattering or anything and no you can't come into my room and take my stuff sort of thing. DD have to learn how to deal with evil people in life, it's sad that she needs to do this at age 12, but heck by age 18 she will be a pro and it will only help her in adult life.

Make it clear to DD, SD likes drama and as long as you feed that craving it will be there, but as soon as you take it away she's a helpless little snot with no personality and no friends.

as you say that you adore her I assume you two have a good relationship until you found this out. Normally I would say it is up to her father but this affects you and your daughter.

I would sit her down with her father there and explain to her how you are hurt by her doing this. Don't get into letting her make excuses etc. Just let her know that you are very hurt and then her father should step in and give her a punishment.

If you want insight, sit your SD down and talk her in a non confrontational way designed to get answers. An honest and open conversation can go a long way to help your understanding. From there you can decide what to do (because things are rarely as one sided as they seem.)

Not just gone.... gone permanently.