And why incestuous when it comes to House?

Well, because there are only six characters and it's all very mix'n'match. Cameron is pining for House, but sleeps with Chase (who "never says no" to House) because "of all the people [she works] with [he's] the one [she's] least likely to fall in love with". House, meanwhile, interrupts Cuddy's date because he allegedly "wants her for himself", while Foreman breaks up with his nurse girlfriend and has heart-to-hearts with Cameron and House. It all stays in the family, so to speak. ;P