Hitsuji no Uta

Both are ovas with incest and tragedy.  

Both shows take a neutral if not sympathetic positions on vampires. They show the hardships in their "life", the emotional damage the hurting or killing others causes, how humans can feel pity with vampires and "feed" them and so on. In short, vampires are portrayed as life different from us, with their own share of problems and not as monsters. Shiki's cast is large and faces a standoff with villagers to the point of outright battle, where Hitsuji no Uta has the focus on a sibling couple and their closer social circle of friends and family. The commonality is the take on vampires, the stories are very different but share the intimate moments of glimpses into what essentially is a tragic fate. read more 

Horror romance anime with vampire characters. Progression occurs very slowly, but ultimately there is some substantial character development. Both also have cool aesthetics, always with dark or greyish colors, and are well-directed. Brother-sister relationships are a theme in both. 

I had the same feeling while watching these animes. both have dark and slow moving stories. While Hitsuji no Uta had kind of like a vampire mystery feel to it, Cossette had this gothic and ghost mystery feel. great animes for the goths and vampire lovers out there.