My answer: Nothing. She's a big girl, and she can take care of and make decisions for herself. She doesn't need me looking over her shoulder in some pretentious, overbearing and patronizing patriarchal "protector" role any more than I need her doing the reverse.
Apparently, however, this was the "wrong" answer. As near as I can tell, I should instead place all responsibility for the event solely upon my friend, whom I should then pursue and confront about it (presumably full of "righteous anger") as if my sister's own feelings and formidable cognitive abilities simply don't exist--or, at least, played no role whatsoever in their doing the tango.
I'll admit, at present and with all I know, I simply can't see this as anything but misguided and rooted in ignorance. However, I'm open to discussing the subject, and I'm genuinely curious about others' thoughts thereupon.