skington

Why are so many people interpreting this as comedy?
In the last panel of the previous comic, Susan is told that Diane existed, and "a vampire attacked her last night for having the same magical affinity as you". Susan then goes wide-eyed.
She immediately asks if Diane is OK; then after more exposition, she gets introspective, talking about how Tedd talked about how the ambient magic would attract aberrations, and how after all this time she really should have expected something like this. She's clearly focusing on "same magical affinity as you" rather than "looks and behaves quite astonishingly like you". Ellen ignores this because she wants to tell her that Diane is her half-sister or something; Nanase cuts in and mentions that there have been seven; and at this point Susan freaks out even more and summons a sword. (Whether it's a magic sword, or a normal sword that's nonetheless glowing because it was surrounded magically doesn't really matter at this point.)
Look at panel 6 again. Susan is freaking out, big time. Even in the final panel she's by no means calm. This cannot possibly be sarcasm.
She was told years ago that it was her job to deal with vampires, she ignored that because it didn't appear to matter, but now that the vampires are here, she realises that she's Buffy before season 1: she has no idea what to do. Whoever Diane is, is totally meaningless in comparison.
A glowing sword suddenly appeared in her hand; so? That's not important. What's important is that she needs to kill all the vampires. The rest is window-dressing.

Why are so many people interpreting this as comedy?
In the last panel of the previous comic, Susan is told that Diane existed, and "a vampire attacked her last night for having the same magical affinity as you". Susan then goes wide-eyed.
She immediately asks if Diane is OK; then after more exposition, she gets introspective, talking about how Tedd talked about how the ambient magic would attract aberrations, and how after all this time she really should have expected something like this. She's clearly focusing on "same magical affinity as you" rather than "looks and behaves quite astonishingly like you". Ellen ignores this because she wants to tell her that Diane is her half-sister or something; Nanase cuts in and mentions that there have been seven; and at this point Susan freaks out even more and summons a sword. (Whether it's a magic sword, or a normal sword that's nonetheless glowing because it was surrounded magically doesn't really matter at this point.)
Look at panel 6 again. Susan is freaking out, big time. Even in the final panel she's by no means calm. This cannot possibly be sarcasm.
She was told years ago that it was her job to deal with vampires, she ignored that because it didn't appear to matter, but now that the vampires are here, she realises that she's Buffy before season 1: she has no idea what to do. Whoever Diane is, is totally meaningless in comparison.
A glowing sword suddenly appeared in her hand; so? That's not important. What's important is that she needs to kill all the vampires. The rest is window-dressing.

That was my point, actually. There's so much natural variation that it would be frankly astounding if either (1) two mothers impregnated at the same time gave birth at the same time, or (2) two mothers impregnated at different times nonetheless varied exactly enough to give birth at the same time. Occam's razor says that if Susan and Diane were born almost exactly at the same time (which may well not be true, of course, and it only needs to not be true for one of them), and they resemble each other, it's because they have the same mother. Having the same father is a very easy next step; having different fathers is unusual but slightly plausible? Having the same father but different mothers, but nonetheless born within 20 minutes of each other? We're getting into "I'm totally a human" t-shirt territory.