I find it odd that a guy would ask about it. What does he expect you to say? Obviously, you love your brother, and you don't want to speak badly about him; why would a date want to put you in such an awkward position? If a guy does ask about it, I would think it would be discussed much further down the line.
Your siblings' lifestyle choices, no matter what they are, shouldn't be up for analyzation on a the first few initial dates; you can't control that, and quite frankly, it's none of his business, until he becomes almost part of the family.
If a date brings it up when there is still yet no "understanding" between you two, try to lightly deflect his impertinence.
First, wait a moment, saying nothing. Maybe then he'll realize the chutzpah of asking such a question.
Then, "How do I feel?" Answer a question with a question. "How anyone else in my position would feel, I guess."
Change the topic and move on.