@hue-man,
The utilitarians just want the greatest happiness for the greatest number of people.
If the five people tied up on the track were Adolf Hitler, Heinrich Himmler, a serial killer, a psychopath, and a half human half demon spawn of Satan, and the 1 person is Mother Teresa or Gandhi, then the utilitarians would say you should let the train slam the five people, because in the long run, the one living produces more happiness than the five.
But all things being equal, if all six produced the same amount happiness, then it's right to only sacrifice one, not the five.'
And therein lies the utiliarians problem, how can you tell whether the five or the one produces more happiness in the long run.
As for the saving the family member against a stranger, the utiliarians would say, "What entitles you to overturn the dictates of impartial and pure reason?" In other words, familial relations play no rational part in morality or moral calculation.