My SIL to be (DB's fiance) is very outgoing and sociable, and in the company of my family at least, I've rarely seen her take anything very seriously.
Having met up with them both recently (me, DP, other DB and his DP, who all share a flat together and try to give each other as much privacy as we can), she hosted wearing the shortest dress I've seen someone wear in real life, and as the night went on, it left nothing to the imagination.
Both my other DB (not her partner) and my DP said later that they were very uncomfortable, but as she fidgeted in her seat, stretched out her legs on the table, jumped up regularly to replenish drinks and had no qualms about bending over right in front of us, nobody said anything right away and it got to the point where it was too late to acknowledge at all, and difficult because she was in her own house.
AIBU to want to avoid going round again, or is there any way of saying something to her or DB that won't cause a row after the event?!