Just invite them to hang with you while you and your cousin ski. Ask if they want to, if they don't, don' make it a big deal. Just have fun skiing and tear it up! I have some friends that aren't in the mood to ski often but when they hang with my bro and I and watch us ski, talk technique and have tons of fun they usually end up wanting to try it. We're always trying to improve and run a better pass and I think that how into skiing we are, no matter how we're skiing, is what draws people into trying. It's all about having fun after all, so make that the biggest point!