Just take your shirt off and tell them to do the same...you'll feel much better

Just tell em "only orange juice"

I'm pretty good at throwing shit back at people.I would of said "If I spent as much time working out as you do sitting on your fat ass watching TV,I'd be twice as big" or "I didn't know it was natural to be so fat you can't bend over and tie your own shoes" or "How many times a day do I need to throw up to be as skinny as you?" You get the idea.Pick something about their body and have fun with it.

my ex "accused" me of juicing last year.... it wasn't intended to be nice or funny or accepting ... at least your family understand and accept you and were what seems like having a bit of fun at your expense. You have to learn how to laugh at that kind of stuff be it good fun or insult as with my ex. I laughed my ass off at my ex and told him he was just jealous that my arms were bigger than his ;-)

The best way I handle it when it happened to me was I turned off the music in the house. When everyone looked at me I put the person on the spot. Hey Jason I overhead you call me a juice head have you seen me juice have I told you I do that?

Then I tell them if you go to the gym as much as you talk trash you probably be my size. Then I would demand an apology in front of the family and tell him some one needs to teach you respect and manners.

Tell them that you have been diagnosed with low t and that yes you are on a prescription dosage of testosterone, a lot of protein and a lot of Creatine. Other than what the Dr has you on you most certainly are not a juice head.
Bro people are jealous people don't understand you could be doing a lot worse than aas. It's hard just try not to let them get inside your head.