“Everyone has intrusive thoughts, but that doesn’t make them bisexual, I think it depends more on what they actually want. But let’s say if you or your sister where bisexual, I wouldn’t care, because none of that stuff really matters.”

Perhaps, in your nephew's mind, he came out to you 5 years ago. And you passed the test so perfectly, you never even realized you'd taken it. In that mindset, he came out to you years before anyone else.

It's not a conversation you can force out of him ... and chances are, the conversation will occur naturally, without you even realizing it's happening ... until it's already happened.

As a side note: In many countries, I don't think it'll be that long before someone is no longer assumed to favour the opposite sex. They are just going to favour the person that makes them feel the happiest.