Try to relax, Alvena. Have you ever gotten so worked-up about a situation only to find that things turned out fine? Well, that’s how you’ll feel about this as well. A good majority of things we worry about rarely happen. I speak urdu at home on a daily basis, but I get nervous speaking in Urdu around people that I’m not familiar with or that I usually don’t speak Urdu to. In these situations, if the person starts speaking in urdu, i respond back in English. And without getting upset, the other person automatically understands that I prefer to respond in English. There, that’s it. Nobody creates a hoop-la over it. Usually my response in English shows that I understood the Urdu correctly. It’s no big deal.
The guy will be able to tell that you can speak Urdu because you said that you’ll be speaking in Urdu to the Aunti/Uncle. Perhaps your mom might speak to you in Urdu as well and that will also be an indication. Just speak to him in English if that’s what you comfortable with. It’ll show you have a decent grasp of both languages. 
Plus, you’ve been raised in the west, and trust me, it won’t come as a scandalous shock to the guests that you feel more comfortable speaking English because that’s the language you have to depend heavily upon outside of your home.