In fictional settings and folklore, vampires and their actions are often subject to various cultural norms and rules that may vary depending on the specific vampire mythology being portrayed. Generally, biting the familiar of another vampire could be seen as disrespectful or even hostile, depending on the relationship between the two vampires and their respective familiars.
Familiars in vampire lore are often loyal servants, followers, or companions to a vampire. They may have a special bond with their vampire master, and their connection is usually sacred. If one vampire were to bite the familiar of another, it could be interpreted as an act of aggression or a challenge to the other vampire's authority or ownership over the familiar.
Of course, these rules are all fictional and subject to the creative liberties of the author or storyteller. Vampire lore varies widely across different works of fiction, movies, books, and other media. Some stories might depict vampires as having little regard for each other's familiars, while others could emphasize a strict code of conduct and respect among vampire clans.
In any case, when dealing with fictional scenarios, it's essential to consider the specific rules and dynamics established within that particular world or narrative. The perception of such an act would depend on the context and the rules of the fictional vampire society in question.