That's a rough one, Billygoat (BTW, why that handle?). You certainly don't want to screw up your relationship with your parents, or with your brother, so you have to be very careful. To avoid the moniker of "apostate" you have to avoid saying anything overtly against the Watchtower organization. You don't have to be nearly as careful saying anything negative about the Bible, God or individuals within "Jehovah's organization". They will "understand" that you're now a worldly person and will likely make allowances. But if they get the faintest whiff that you know something about the JW organization that you can't get from the newspaper or from regular WTS publications, they'll smell "apostate" and likely shun you.
The best approach, I think, is to be alert for situations with your brother where you can talk a bit about things that, if he investigated, would cause him to come up short and spark curiosity about why JWs believe what they believe. What you say depends entirely on his and your background and interests. If you have an interest in science, that's an excellent way to go, since the WTS teaches a lot of nonsense with respect to it.
LDH's suggestions are excellent. Beyond what we've said so far, I think we'd need more detailed personal info in order to make more specific suggestions. What are your and your brother's areas of interest, so so forth?
AlanF