Incest is only a problem in that the genetic info is too similar between the parents, and so there is a much greater chance of defects becoming active in the offspring.
Blood itself or bloodline is not the issue.
The greater the distance between one and one's partner the less similar that genetic data is, and the lower the risk of a defect appearing in the your offspring. Some cultures allow cousins, others not. It's a subjective opinion where a culture draws the line.
Ultimately we are all related (if it comes to that all living things are) but it doesn't count as incest if one is outside the legal definition.
Besides there seems to be a natural distaste for most to be intimate with those from one's own family; probably a case of being too familiar and seeming at best a friend rather than a sexual partner.
Naturally this is quite aside from any religious teachings. It's a corporeal issue rather than a religious one.