I advocate asking for an attorney immediately upon LE saying they want to ask questions, no matter the circumstances. Perhaps all the more so in cases where family members are automatically persons of interest.
Maybe that's because I've simply read about/watched too many cases in which LE went after the innocent, based on little more than gut instinct and/or an unshakable belief that someone wasn't acting "right." Personally, I'd rather risk looking guilty to LE/the public than to expose myself to potentially irreversible legal jeopardy.
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About SP's "brand." Since I think (I think I think, that is..

) that SP was taken & held by person (s) for a personal reason, AND that her captors (s) were paralyzed by indecision how to extricate themselves from a situation they hadn't anticipated (intense local then national interest & large rewards being offered), AND that the "brand" was seemingly unreadable, perhaps even no more than random markings, what makes sense as a possibility IMO is that her captors were listening to the news, heard the speculation by many that SP has been kidnapped by sex traffickers, looked up or knew that traffickers often branded their victims, and "branded" SP to make it look like that was why she was kidnapped.
---According to KP (LE did not support or refute), SP's "brand" had raised scabs. If so, the (cuts?) weren't inflicted within a few days of her being found...
---If the "branding" was done to suggest sex trafficking, maybe SP's captors also deliberately chose a random pattern to make sure they didn't accidentally copy the brand of folks they wouldn't want to antagonize.