Liangelo is an adult and screwed up his chances on his own. Considering how sucky he is at basketball, he's lucky to be getting paid by anybody to play.
Lamelo probably wasn't going to be cleared to play ncaa basketball, so playing pro ball overseas isn't a bad option. His father is wealthy and his brother is a millionaire. If basketball doesn't work out, he could come back, finish school, go to college and get a job. Not exactly a catastrophe. Yeah, he might miss his high school friends. Or he might not. Or he values playing basketball above that.
They're playing in a crappy league because no good league would want an 18 year old bad player, who is dumb enough to steal in China, alongside his 16 year old, unproven brother. With playing in college not an option, this could have been the best alternative.