Disclaimer: I am not JK. Obviously.
"Andi! Andi! Where are you going?" Narcissa cried, following her sister to the front door. "Andi, you can't leave us!"
Andromeda turned around with her hand resting on the ornately carved doorknob. A look of pain and sadness, but determination, crossed her beautiful face. "Cissy, this is the hardest choice I've ever had to make. I don't want to leave you. I love you – and Bella. But I love Ted and this is the only way I can be with him. Mother and Father would never allow me to marry a Muggleborn."
Bellatrix was standing a few paces behind Narcissa, a look of fury on her face. "So you're betraying the noble and most ancient house of Black and deserting us and marrying a filthy Mudblood?"
"Don't call him that" Andromeda said in a deadly voice.
"I'll call your precious Muggle spawn whatever I like, you little blood traitor!"
"Bella! No!"
"Shut up, Cissy!" Bella shrieked, eyes nearly popping out of her head, making her look deranged. "I have made a respectable pure-blood marriage. I may not love Rodolphus, but at least he is a pureblood wizard! Don't you understand, Andi? It's not about love! It's about raising pureblood children and continuing the family name and ensuring that our family tree is not tainted with the spawn of Mudbloods and blood-traitors!"
"No! No, Bellatrix! It IS about love! Love and family are the most important things in the world! I love Ted and I will not desert him!"
"But you WOULD desert your own family!"
Andromeda had tears running down her face. "You don't understand" she sobbed. "None of you understand. I have never agreed with the beliefs of our family — the obsession with blood purity, for example. I don't belong here. I belong with Ted."
Narcissa seized Andromeda's arm. "Don't talk rubbish, Andromeda Black! Of course you belong here! This is just a phase you're going through, I'm sure. You'll get over the Mudbl— I mean, Ted — soon."
"It's not a phase, Cissy! I am in love with Ted, and we are getting married in September!"
Those simple last words had roughly the same effect as a bombshell would have had if it had landed on them at that very moment. Bella screamed in outrage and lunged at Andromeda, Narcissa gasped and staggered against the wall, white as chalk. The door at the other end of the hall flew open with a crash and Druella Black burst into the hall, demanding to know what was happening. In about five minutes of shouting, banging and crying, Druella had been fully informed of what her second eldest daughter meant to do, and rounded on her with such fury that her resemblance to Narcissa became even more pronounced. Andromeda was being held in an iron grip by Bellatrix while Narcissa stood tearfully behind Druella.
"You just wait until your father gets home, Andromeda Cassiopeia Black" Druella hissed. "Never have I heard of such treachery in all my life! You'll be sorry one day, girl. You'll come crawling home, begging for my forgiveness. One day that filthy Mudblood will hurt you, and you will be sorry you left your family. You know what? I'm not going to wait until your father gets home. Just go. Leave. You want your filthy Mudblood boyfriend so much? Then go and join him!"