I'm sorry this has happened MissFenella. Massive hugs to you, this is very hard to deal with and also very isolating. This is something I do have experience with, but I will PM you
Do you feel she is acting in a very predatory manner? Personally, I would be highly worried if her behaviour includes things like - trying to get her sister to 'not tell mum and dad', trying to bribe her into it, threatening her if she does not comply and so on. Whilst quite a large proportion of abused children do have sexualised behaviour (including between siblings) it's more worrying if they've picked up on aggressive, secretive, grooming, coercing or other very predatory behaviours. It's obviously much more dangerous for other children if there is an attitude of 'I know this is not allowed by mum and dad, but I'm going to do it anyway, I'll just make sure i don't get found out'. I hope this is truly only the second time, but be prepared for it to not be the second time.
You will need to phone SS obviously, I can't remember whether you have legally finalised the adoption yet or not? If not they will be more involved than if you had, because they are legally responsible for her. Your older DD does need therapeutic input but getting it can be hard. If she is acting in a predatory manner she needs therapy even more urgently, don't wait, in under 2 years she will be 10 and if she does it to another child then you may wind up with police involvement. Fight for her to access some form of help, such as referral through to CAMHS. Attitudes among professionals vary though - you may get concerned and helpful professionals and I hope you do, but also be prepared for unhelpful or frankly stupid people.
At home, how are you dealing with this? It terms of safety, do you have a safety plan? Eg. Elder and younger DD will never be left alone together, one child at least must be under line of sight supervision at all times. Not permitting elder DD to go to friends houses and line of sight supervision with friends who come to your house. When friends came over here, I had additional rules, no playing hide and seek ever being one of them. Close supervision is hard to implement and I can't pretend it isn't frustrating, but it may be necessary. Do they currently share a bedroom? If so, I would seriously suggest seperate bedrooms.
Also, younger DD needs some talking to. Empowering her to be able to come tell you if this ever happens again, and encouragement to talk about it. The message of - we talk about sex in this family, nothing is a secret.