Your "sister-in-law" told you? This is your problem. You need to talk to your wife. She has fears, concerns, questions. YOU can answer her questions, calm her fears and ally her concerns. It's on you, not her. Just talk.
Yes, you are. BECAUSE a woman has no foundation about crossdressing, something that you and I and most crossdressers have had decades to get used to, a woman will naturally ask about homosexuality or sex change. This is perfectly reasonable and expected given the shock of learning about their SO's crossdressing. How could you possibly blame any women for asking such a basic question? I think you are quite insensitive to what any woman is going through. WE
owe it to our SO to listen to any question and answer honestly and not defensively. We're the odd ones here, not them.