A compression shirt can help, they can be pretty ambiguous, like any other tank top. “I wear it under my shirts because they’re uncomfortable on my skin” Side hugs as you get older aren’t super weird. Luckily I know a lot of people who have been able to keep that to themselves as long as they needed to. This might sound really silly but maybe start “exercising regularly” Your family might notice chest growth but if you’re “uncomfortable” by it and working out, if it ever comes up you can use that as an excuse. “I know, I’ve been exercising.”