Until you turn eighteen, it sounds like they have the authority over you, not your mother. It sucks, no doubt about it. If you spend the night at your BF’s house again, be upfront with them about it. If they plan on weaponizing the Sheriff’s office, let that office know it. Call them before you spend the night and tell them what's up. That you are fine, and they'll call to try to force you home. It depends on the department if they will fetch you or not. If it's a high crime area, they'll probably just roll their eyes unless they think you're in danger.

If the grandparents are going to call around all your friends, let key people know ahead of time. All it takes is a few friends who know what's up to comment on social media if a post goes out that “OP is fine, her family is Extra. Ignore them.”