I totally understand why everyone prefers to make this Lily’s best friend. Lily didn’t have any known friends, so when Marlene McKinnon was mentioned in OotP, the fandom collectively decided to make her Lily’s best friend.
I really do get it, and I don’t mean to ruin headcanons and rain on parades.
But I think it’s more likely that the two weren’t that close.
There are four mentions of her in the books:
PS:
No one ever lived after he decided ter kill ‘em, no one except you, an’ he’d killed some o’ the best witches an’ wizards of the age — the McKinnons, the Bones, the Prewetts an’ you was only a baby, an’ you lived.“
So the “McKinnons” are mentioned as the best witches and wizards of their age. This probably doesn’t mean just Marlene. It’s a lot more likely that it means Marlene+husband or Marlene+sibling.
GoF:
“Yes!” said Karkaroff. “There was Travers – he helped murder the McKinnons!
So from this we can assume that the McKinnons were well-known enough that their murder would be something people would remember. Maybe it was particularly public because it was an entire family, but Karkaroff believes that it’s enough to shift the focus to Travers.
OP:
“There’s me,” said Moody, unnecessarily pointing at himself. The Moody in the picture was unmistakable, though his hair was slightly less grey and his nose was intact. “And there’s Dumbledore beside me, Dedalus Diggle on the other side… that’s Marlene McKinnon, she was killed two weeks after this was taken, they got her whole family.
This quote strongly implies that the entire family was killed in one swoop. It’s hard to know what “her whole family” means. In the case of Lily/James, most people say Lily/James’ deaths. The fact that both Karkaroff refers to them as the McKinnons and Moody says they got Marlene’s whole family, I would say that “whole family” would include at least 3 people if not more. It could be Marlene’s parents, a sibling, and Marlene herself. Or it could be Marlene, her spouse, and her child(ren).
If Marlene is married, it’s likely that Marlene McKinnon is her married name. Because if Marlene McKinnon had married John Smith but kept her name, Hagrid would have said “Marlene McKinnon and John Smith” or used “the Smiths” to (incorrectly) refer to their entire family.
DH:
Wormy was here last weekend. I thought he seemed down, but that was probably the next about the McKinnons; I cried all evening when I heard. Bathilda drops in most days, she’s a fascinating old thing with the most amazing stories about Dumbledore. I’m not sure he’d be pleased if he knew! I don’t know how much to believe, actually because it seems incredible that Dumbledore-
This to me is the strongest indicator that Lily and Marlene were not close friends. I know that everyone expresses emotions differently, but Lily sandwiches a mention of her best friend’s recent death in between a comment about Wormtail and some light gossip about Dumbledore. And it’s clear that this is the first time that she has discussed the McKinnons with Sirius, because she tells him how she reacted when she found out. So if they were long-time best friends, she’s being really cavalier.
If Ron had died, Harry would not write a letter to Hermione saying:
“Neville stopped by today and seemed down, but that was probably the news about the Weasleys. I cried all evening when I heard.”
Hermione would not need to be told that Harry was upset over the Weasley’s deaths, she would be mourning right along with Harry. And Harry would not drop a mention of Ron’s death into a casual thank you note. If this had been the first time he had communicated with Hermione since the Weasleys’ deaths, he would either go into at least a bit more detail or omit it entirely. And he would show some sign that he was concerned that Hermione might be mourning as well. Even if he had magically come to terms with it within 24 hours, he would not drop it into a letter to Ron’s other close friend.
Obviously, you’re free to ignore this and continue shipping Marlene/Sirius, I’m not going to criticize. But I think fanon can muddle our understanding of the characters, and honestly even I was surprised at how few times she’s actually mentioned.
So just know that if you’re writing a fic about Lily Evans, her best friend doesn’t actually have to be named Marlene.
I mostly agree, but I don’t think that Marlene and the rest of her family were necessarily killed all at the same time.
It’s the most obvious interpretation of what Moody said about them, of course (I didn’t even think that much about it until reading this post). But if Marlene was killed two weeks after the photo was taken, it would also mean that Lily wrote the letter 2-3 weeks or a month at most after that. Considering that by the time she wrote it, Harry was old enough to fly on a toy broom, the Potters must have been in hiding for some time already. Meeting with the rest of the Order for the sake of a group photo would have been too much of a security risk, so I’m assuming that a lot more time passed in between.
So it’s likely that Marlene was murdered first and the other McKinnons later towards the end of the first war. I think Lily’s reaction is reasonable if Marlene was a friend (though maybe not her best friend) who has been dead for a while, and now the rest of her family (whom Lily possibly didn’t know very well) have been killed too.