Sampson, along with Gregory, are servants of the Capulet family. Both men appear in Act I, Scene 1 in a confrontation with the Montague servants which begins half in jest but quickly escalates in seriousness. It is Sampson who instigates the fight, purposely insulting the Abram, a servant of the Montague family, by "biting his thumb" at him. The altercation quickly spreads to involve other servants and friends of both the Capulets and the Montagues, and serves to clearly illustrate the atmosphere of tension and bad feeling which exists between the two prominent families and is the basis for the tragic tale to be told.
Generally, Sampson is listed with Gregory in the list of characters, and the 2 are often described as such:
"Two servants of the house of Capulet, who, like their master, hate the Montagues. At the outset of the play, they successfully provoke some Montague men into a fight."
So, your question is a good one, and comes with a simple and straightforward answer: Sampson is a Capulet. Perhaps not by blood, but bound by his word and his work. And, remember, Juliet is our Capulet, while Romeo is our Montague.