From my experience, stuttering is caused by some sort of brain activity that is not letting you think enough about speaking. In my experience, that activity is social anxiety. You probably don't stutter with friends because you're their homie and you guys are cool. Family/strangers might make you a bit more stressed and cause your stutter. Advice? Just keep on running experiments on yourself, see what gets you stuck, and work on getting more familiar with that person/situation/etc.