Right.
So, here's the thing. I have a sister who is nearly fifteen years younger than I am. Further, she was adopted from China.
If, when my sister was 13, my brother (would have been 30), myself (would have been 28), and my sister (13) were on a road trip, and decided to save money and just get a quick nap in at a highway rest area, would you think the police ought to cuff my brother and I and send my sister to CPS until everything was 'sorted out'?
At age 13, she did not have any photo identification. My brother and I would not have had any note whatsoever, certainly not anything notarized, and my parents are the types who hardly ever answer their phones.
But, according to you, this would be the right thing for the police to do. Would it be the right thing if my little sister were white? Then, it would just be two white guys and a much younger white girl. Would it be the right thing if my brother and I were black, and my sister were white?
Sure, the vast majority of siblings are closely connected in age (and, share the same skin color). Yet, my sister and I are not; my mother is 9 years older than her brother; and, my father, who is one of ten, is seven years younger than his oldest brother and eight years older than his youngest brother (seven years older than his youngest sister). The age gap is unusual, but it is not criminal and it does not warrant cuffing and detention.
In fact, were that situation to happen, I would patently refuse to be placed in handcuffs, the result would probably be the use of physical force to place me in handcuffs, and I would respond by suing the police department.
Thus, even the CYA angle on this one is ridiculous.