The average human has around 5 quarts of blood and can loose 30% of it and still live. Since Aya is a tengu and spends a lot of time at high altitudes she probably has more then this.
Actually, with Aya and Remilia's dialogue in Scarlet Weather Rapsody, I wouldn't call this pairing that unlikely. They certainly act very familiar towards each-other, at least *^_^*
And of course, Remilia's comments about "eating" Aya when she is playing a monster's part can carry a double meaning as well..