How come i have altitude illness, but nobody else in the family has it?

Not genetic: Altitude illness is not genetically determined.

Non genetic causes:: More likely to occur if you are more active, dehydrated, or had less chance to acclimatize. Sometimes younger people at higher risk because move around more ; more apt to have relative oxygen deficit--breathing more deeply with air hunger. Another group which struggles at first are those with sleep apnea which worsens at altitude, w/ sleeplessness, headache ; nausea. Those w/ anemia also > risk.