OP, given the information you have given, I would try telling her all, or some of the following …..
Reiterate, in a loving but firm way, all the new rules, explaining they are to protect you all, not just her.
Tell her you love her just as much as ever and just as much as you love all your daughters and that you only want the best for her. And will never stop supporting her as long as you can. But because of the discord, you now see that there needs to be these boundaries to help support you all.
Tell her she is an adult living in a home with four other adults all deserving equal consideration. Not a hotel where one person demands and dictates who she eats with and where her meals are served. Tell her she, along with you all, is lucky to have such a supportive home but if she or anybody else feels there is a better deal elsewhere, then she isn’t a prisoner there and doesn’t have to stay.
Tell her you can no longer accept that she has a choice to be vindictive in your house where you, not her, oversee the household harmony, and you won’t allow her to continue to unkindly single out DD1 as unworthy. Tell her you won’t be accepting unkindness or bullying or squabbling from anybody else either.
Tell her she is no longer a child and you all now need get along together with mutual respect and kindness as a family and fit in together so, for the sake of all of you, not just her, this is now a fresh start
Tell her that eating together around the table where no negativity is allowed, only a safe haven of happy chat, debate and news sharing, is entirely normal and will help prepare her for what it will be like for her as an adult in the real world, whether in a job or at college.
Tell her she needs to be able to get along with others and learn to navigate her way through life without tiresome unnecessary dramas and this is to support her to do that.
Tell her that if she doesn’t want to chat, she doesn’t have to, she can just listen.
Tell her that she if she doesn’t want to eat, then she doesn’t have to, she can just listen.
But it is the expectation that meals will be taken all together around the family table, in a normal civilised way from now on with good manners. No excuses.
Tell her that if anybody wants to argue or make a fuss or act in anyway unkindly, they must do it away from your home as you will no longer be made complicit nor tolerate it.
Tell her that now all your children adults, you have a right to expect peace in your home and your expectation is that everybody makes a real effort from this new fresh start. All arguments left behind.