This reminds me of that doujinshi, "Scarlet and Scarlet"

Anyway, I kinda remember something about vampire and rain don't mix or something? Is Remi and Flan an exception to that as well?

They arent an exception at all. This is either artistic liscence or the artist is just saying that Remilia is in powered up mode.

Patchouli herself used a Rain spell to keep Flandre in the mansion. If rain didnt have an effect on her, Flandre would have just busted out. So weakness to rain is diffenetly a canon as far as ZUN's vampires go.

I only give a small nod to Remilia since its possible Remilia could have learned how to either push through Rain or maybe she learned how to not totally crumble in it.

I wonder what happened to Remi's arm...

I think Remi and Flan fought and the rain started. From the look of Remi's wings I think she is sheltering her sister

Krugger said:
From the look of Remi's wings I think she is sheltering her sister

I just noticed that from the wings angle.

@SharkGears : You're right. For a while I've been wondering where does this deja vu come from. I checked my doujin folder and found it.