I am confused about the marriage of Harry Potter and Ron's sister, and why he didn't marry Hermione. Any idea?
J.K. Rowling says that Hermione and Ron's relationship was not credible
In an interview with Emma Watson, who played Hermione Granger in the Harry Potter films, Joanne Rowling stated that the reason for the Hermione/Ron pairing (and perhaps by implication Harry and Hermione not ending up together) was "wish fulfillment."
“I wrote the Hermione/Ron relationship as a form of wish fulfillment,” she says. “That’s how it was conceived, really. For reasons that have very little to do with literature and far more to do with me clinging to the plot as I first imagined it, Hermione ended up with Ron.”
"I can hear the rage and fury it might cause some fans, but if I’m absolutely honest, distance has given me perspective on that. It was a choice I made for very personal reasons, not for reasons of credibility. Am I breaking people’s hearts by saying this? I hope not.”
She also says it was not for reasons of credibility, implying that she did not see the relationship between Hermione and Ron as credible.
That said, Ron and Hermione still canonically had feelings for each other. Note that JKR is not saying that their relationship couldn't have happened, merely that it would be difficult for it to work out. Plenty of incompatible people end up together.
However, a Harry/Hermione pairing is also not credible
Harry and Hermione never showed the slightest bit of romantic interest in each other in the books. Even if Hermione and Ron's relationship may not have been likely, as JKR has admitted, that does not imply that Harry and Hermione would get together.
"After you left,” he said in a low voice, grateful for the fact that Ron’s face was hidden, “she cried for a week. Probably longer, only she didn’t want me to see. There were loads of nights when we never even spoke to each other. With you gone . . . ”
He could not finish; it was only now that Ron was here again that Harry fully realized how much his absence had cost them.
“She’s like my sister,” he went on. “I love her like a sister and I reckon she feels the same way about me. It’s always been like that, I thought you knew.”
—Harry Potter and the Deathly Hallows
By contrast, a relationship between Ginny and Harry, who had genuine, strong feelings for each other, and no history of animosity (in contrast to Ron and Hermione) is credible.
Ginny and Harry have a good relationship. A relationship between Harry and Hermione would require more than Ron and Hermione lacking a healthy dynamic; it would require actual romantic feelings between Hermione and Harry, which never existed.
Even if Ron and Hermione always fought, they were well paired in the worldly sense.
Even if Ron and Hermione weren’t together, Harry and Hermione would have a very dull romance as it was more like brother and sister.
Let us assume for a minute that JK didn’t write any chemistry between either of the pairs.
Now, based on the fights alone, we see when Harry and Hermione didn’t talk, it was just a boring normal situation of 2 friends fighting and then patching up.
But when Hermione and Ron fought (even before having fights due to jealousy or as a couple….), it was like a 3rd world war. At least Ron and Hermione had sparks flying, even if it was in a fight. It was almost funny.
Both Harry and Hermione were nerds whereas Ron was funny and charming and Ginny was hot.
Don’t get me wrong, Hermione is the embodiment of a great best friend as well as an awesome person and a great character study.
Also Harry was already weighed down by responsibilities and tensions. He doesn’t need a librarian however bad-ass she might be.
In life even your closest friend cannot satisfy some qualities coming to love life. Harry and Hermione needed someone who could bring color and variety to their lives.
You can never have a perfect pair without a few thundering fights which was just the case for Ron and Hermione.
Also it does help that this way, all the main characters get somehow related to Weasleys and purebloods in the canon.
Oh and btw if somehow subconsciously, Harry was inspired by his own parents, Lily was more like Ginny than Hermione in personality. Ginny is very smart and ferocious too.
Coming to storyline, when Harry was setting off on his own in 6th part, Hermione and Ron accompanied him, true. But that’s because they were fighting in the war too and not for some romantic reasons.
Now in the case of Ginny, even if she didn’t accompany him, she stood strong for him. She didn’t tell him to be careful or some huge drama which Harry doesn’t need but was a calm and composed strength waiting and giving him something to come home to.
Not many are like that. They would either be scared and not get involved or create a huge drama (Lavender or Parvati(no offence they turned out fine in the end too)).
Its just unfair to say that Ginny wasn’t involved in many fights or scenes. They just didn’t go in the same circles as they were not in the same class. One must not simply forget that best friend and lover are not always the same.
Why I always thought that Harry and Hermione would be a good pairing personality-wise (although, as pointed out, it would be completely non-canon):
They make a great team. Harry and Hermione always seemed to be the core of solving the mysterious events surrounding Hogwarts: Hermione with knowledge and wit, while Harry - with wit, vision and intuition. They seemed to understand what the other is thinking and be able to respond to it, and I find this unspoken mutual understanding very attractive. They seemed to complete each other so well. Harry had the guts, bravery and impulsiveness, the hot-blooded 'do', while Hermione had the wisdom to point him in the right direction, the 'wait! Consider this before you do something'. This mutual understanding they had, in combination with those qualities, to my mind, personality-wise, could potentially make them a great couple, able to overcome life's challenges and move forward while also enjoying each other's company.
Hermione and Ron, on the other hand..well, yeah, they make a very down-to-earth couple who have a lot of quarrels need to talk it all out in order to fully understand each other. By the vibe I get, (personality-wise) Harry and Hermione come way more naturally while Hermione and Ron .. really need a lot of work to work out as a couple. (Harry and Hermione also seem to feel really comfortable around each other, while I can't say the same about Hermione and Ron.)
Somehow I get the feeling that JK Rowling poured her own unrealised wishes or some hurt about how relationships don't always work out the way you want them to into the story. But that's just my impression, perhaps as a psychology student, perhaps as an artist, perhaps as someone with a different view of how the pieces of the World fall together.
That said, I must admit, up to this point, I've only seen the HP movies, therefore my judgement is biased and based solely on the actors' portrayal of the characters. I might be missing something vitally important.