Well, Harry would be quite taken aback. He'd have to reevaluate his relationship with Hermione. He might worry that her loyalty has shifted. But at the same time, he would also see it as an opportunity to perhaps get some inside information on the Slytherins. Ron, on the other hand, would be extremely confused and probably not very happy about it. He might feel like Hermione is now part of the 'enemy' camp. However, as time goes on, they might come to accept it, especially if Hermione shows that her loyalties still lie with them. But it would definitely cause a lot of tension and confusion in their friendship at first.
Harry would be really shocked. He's used to seeing Hermione as his friend and fellow Gryffindor. If she was suddenly Draco's sister, he'd be very suspicious at first. He might think it was some kind of Slytherin plot. Ron would be even more flabbergasted. He has his own issues with Draco and to think that Hermione was related to him would be hard for Ron to accept. He might even be a bit jealous that Hermione now has this connection to a pure - blood family.
Ron would be furious. His best friend being 'stolen' by his former rival would make him feel a mix of anger, betrayal, and jealousy. Harry would probably be in a difficult position. He's friends with both Ron and Hermione, so he might try to mediate at first, but if Draco is being difficult, he'd side with Ron. Ginny would be protective of her brother and would likely be very critical of Hermione for choosing Draco. The Slytherins, on the other hand, might be initially shocked but then accept it as Draco asserting his power and getting something he wants.
Harry could be initially shocked but then very happy for them. He might see it as a sign of hope in the midst of all the darkness. Ginny, on the other hand, could be a bit jealous at first, but then come to accept it and even give Hermione some relationship advice.
The Death Eaters would probably be divided. Some might see it as a great opportunity to gain more power through this union, while others might be suspicious of Hermione and worry that she will be a threat to their cause. The students at Hogwarts would be in an uproar. Hermione's friends would be ready to fight for her freedom. Ginny, for example, would be a strong advocate for Hermione. And Neville, who has grown so much in courage, would also stand up against this wrong - doing. There would be a lot of tension and drama within the wizarding community as a whole.
For Harry, it would be a big shock. He might be more cautious around Draco at first, but as he realizes Draco is now family to Hermione, he could try to be more understanding. With Ron, he'd probably have a harder time accepting it. Ron has always had a bit of a rivalry with Draco, and now that Draco is related to Hermione, it could create some tension as Ron tries to come to terms with it.
Harry would likely be very protective of Hermione. He's her best friend, after all. He might confront Draco immediately and try to get Hermione to see that she doesn't deserve this. Ron would probably be angry too, and might even be more impulsive in his reaction, perhaps getting into a fight with Draco.
Well, Ron could get angry. He has a strong loyalty to Harry, and seeing Harry with Draco, who he has always seen as an enemy, might make him feel betrayed. He might immediately jump to conclusions and start shouting or demanding an explanation from Harry.
If Hermione were Harry's sister, Ron might be a bit more intimidated at first. He has always had a crush on Hermione, and this new connection could make him feel like he has to be on his best behavior. However, as time goes on, he would probably get used to it and their relationship could become even stronger, as Ron would see Hermione as an even more important part of his life through her connection to Harry.
Harry would probably be furious. He's always been protective of Hermione, and he'd see Draco's actions as completely unacceptable. Ron might be a bit more vocal about it, perhaps even getting into a fight with Draco. Ginny, on the other hand, would likely comfort Hermione and be very supportive. Neville could be shocked and disappointed in Draco, as he's a character who values loyalty.
Harry would be completely shocked. He's known Hermione as his best friend and to see her with Lucius, who was once a Death Eater, would be almost unthinkable. He'd probably try to convince Hermione that she's made a huge mistake.
Often, it's portrayed as a slow - building process. Draco has to overcome his pure - blood prejudice, and Harry, Ron and Hermione have to look past Draco's past behavior. In some fanfictions, a common enemy or a magical mystery brings them together. For instance, a cursed object in Hogwarts that only they can solve the mystery of.