All you need is lemon juice, ice, sugar, egg white and a blender (and the pisco, of course). There are several recipes but I find the most basic the best. I second the question about local pisco; I haven't seen it at the LCBO in years.
Did your in-laws bring up the Pisco too? I've been looking Pisco for a while now (read years). The LCBO site doesn't show any in my area. I like it in the traditional sour but it makes a really interesting replacement for Tequila in a Margarita.