You shouldn't. Hey, just because I haven't experienced something doesn't mean it's not true. Your nephew, the guy who spoke in tongues, and their whole church were no doubt having a real experience of God at that moment.
My point was that a lot of the people who appear to be praying in tongues may indeed be having a close connection with the Lord at that moment, but that doesn't necessarily mean everybody else who hears the babbling is being edified. Sometimes it really is just babbling to express religious joy, not a foreign language.