How to tell a manager you need to leave out of country for family emergency?

I have an interview for a job at Loft outlet on wed and I am leaving out of the country onn fri because my uncle has ped and I need to help get everything out of his house but I am not sure how i should tell the manager that I wont be in town for 3 weeks! like im afraid it that it will jeopardize me receiving the job! -__-