no, your SIL was wrong. when asked to bring something then you bring a ready-to-serve dish. preferably in a serving dish, and if possible at the correct serving temperature.
reminds of the time my step mother *insisted* on bring chicken for a family dinner because "she wants to help me", i finally gave in (i learned after that), so my father showed up at my house 2 hours before the dinner, with RAW chicken all spiced and in a cooking bag with instructions to roast for 2 hours at whatever temperature. i was so upset - she said that she wants to help me - but that wasn't "help" it just made things more difficult for me (besides she didn't even think to ask if the oven was free, but that was a whole other story).