Suuuuuuuper random headcanon that literally just popped into my head all of a sudden…
Molly and Arthur were both married to other people with kids before they found each other and married. Bill and Percy were Arthur’s kids from his previous marriage, Charlie was Molly’s. They had Fred, George, Ron, and Ginny after they got married to each other. Molly and Arthur had fooled around a bit and dated a little in Hogwarts before going their separate ways. When they ran into each other by chance, they just clicked very well and this time the relationship lasted.
I imagine Arthur’s first wife died suddenly (either in childbirth or shortly after Percy was born), leaving them all shocked and feeling lost. Whereas Molly married rather young and her and her husband never really talked about some things they should have. Molly always wanted a large family, and she was so excited when she got pregnant. Turns out her husband didn’t want kids at all, leading to a nasty divorce. He signed away his parental rights, which at least made it easier - when Molly and Arthur married later, they adopted each other’s kids.
Also, Arthur is probably white (his family has probably been in Britain forever), while Molly has Romani ancestry (probably mixed with Italian - but she’s not considered Romani herself because somewhere a few generations back, her ancestor married out of the community). Arthur’s first wife was Pakistani and Molly’s husband white (maybe French - and maybe this plays into why she’s got such a strong bias against Fleur when she and Bill begin dating - some unresolved feelings about her past). So you’ve got this mix of kids with all these different looks but who are all related and do have some strong resemblances to each other. Bill, Charlie, and Ginny are all slightly darker than their siblings. Bill, Percy, and Ron are still the tall, lanky ones, while everyone else leans towards the shorter and stockier side. Percy takes after his mother in many ways, and he struggles with never really having gotten to know her; those two things combine and make his relationship with Molly strained all the way until after the War, though she’s always accepted and loved him just as her own (even though Bill was older, he had an easier time accepting Molly as another mum and referring to her as such, but it took lil Percy longer, and Molly legit cried when he did so, and she didn’t bother trying to hide that joy).
The kids are still on the younger side when Molly and Arthur blend their families (Bill 8, Charlie 6, Percy 2 or thereabouts, anyway), and it helps that Bill and Charlie just immediately click together and become best friends, plus Charlie’s been asking for a little sibling. And Molly gets pregnant so fast, and they’re all just excited for the new family member(s!).
They all know they’re a blended family, but it’s not something that’s typically focused on or at the forefront of their minds or anything. But once a year on the anniversary of Bill and Percy’s mom’s death, they and Arthur go and visit her grave and spend the day with each other, reminiscing and telling Percy stories. Until they’re ready to share their grief one year, and they start telling the other’s about how great and kind she was, and then all of them are going to visit her grave and pay their respects. Special time is still given for Arthur and Bill and Charlie, but they all begin to feel like they knew her and over the years, they all just naturally check in with the site and make sure there’s flowers there and it’s looking nice.
But all in all, they’re really the same Weasley’s we know from the books.