This is actually a story of my older sister. When she was just 18, she got herself into a relationship with a boy of the same age. Unfortunately, it was a very rough relationship because the guy was involved with illicit drugs -- both as a user and as a dealer. Naturally, the whole family objected to their love affair. After about 2 years, they separated for good due to misunderstanding and irreconcilable differences. This is a case of a strong love for the wrong person.
It happens to a lot of persons out there. There is nothing wrong with love per se, but sometimes people are just blind when in love. Glad to know your sister finally left him. I cant see any good future to be with him.