In itself, treating your relatives nicely because they're family doesn't seem to sound too bad; it sounds like the obvious and natural thing everybody would do. The problem I have with it is that it means you're intentionally treating everybody else less nicely because they're not family, which to me is a very weak reason to withhold your good will. When taken to the field of real-life decisions, it takes the form of nepotism, which can be seen as bigotry against the entire rest of humanity.