During my basic training, we had several "conscripts" or fellas that were given a choice between jail or the Navy. Obviously, they were compelled to enlist and because of that, they never really gave the military the chance it deserved. What happened to them after that I don't know because unlike other armed forces, your chances of ever seeing a fellow recruit are slim and none in the Navy.
I do know that they were the trouble makers who needed constant supervision, constant reminders of how to perform their duties and constant attention from the rest of us to teach them that we are not individuals, but 76 men, all working to the same end, Graduating...one bad apple can force an entire company back to week one! Those were the guys that got "short sheeted", wire brush showers and cleaning the crappers with their tooth brushes.
Now in 1975, the Navy basic was no tougher physically than high school football practice, it was a mind game. Find the weakest link and force them to break, find the button of weakness in a person and keep pushing it till they break...
Naval crews aboard ship are cramped together in small spaces for months at a time and being tolerant of others is the mainstay of Naval life. I never served aboard ship for my enlistment but was sent to some of the more remote places on the planet, either way, working together was paramount in our training.
This relative may be better served in another venue other than the armed forces, forcing this person into a life unlike most, could be far more harmful than letting he or she make their own mistakes and learn from them. There are always reasons for any behavior and maybe this person is just looking for help and doesn't know how to ask for it.
Back in the day, if you washed out after about week 6, it was a "General Discharge" and that can have a major effect on the rest of a young persons life....