When two things are very similar, and it's hard to clearly define the difference between them, you can use this phrase:

There's a fine line between drinking socially and alcoholism.

There's a fine line between helping your kids with their homework and doing it for them.

There's a fine line between dressing a little sexy and looking trashy.

Usually, one of the things is good or OK, and the other one is bad. The bad one comes last.