I know I might be shunned for saying this but when I first started playing Touhou, I thought Remilia, Flandre and Rumia were related due to the hair ribbon being the same.
The truth is that Rumia is really the future lovechild of Marisa and Flandre. Being a half-vampire she's only partially weak to sunlight. Also, since half-whatevers always inevitably end up stronger than either parent, she was sealed and booted into the past by a third party who feared her potential.