What exactly did i do wrong in this situation?

my gf has moved out of her moms house and into her dads(shes 15). shes had home issues with her mom for a while and needed to get out. she was depressed about it for a while, until about 2 weeks ago so i thought it was good and over. Plus i had thought that all of her friends knew about that change(this part had relevance). Anyway, today we were all jus talking. her friends, her sister, my friends, and me were all jus hanging like usual. so were talking, then i go and ask her sister hows life been since your sister moved out. i guess thatwas a mistake since my gf yelled "omg!" and stopped talkin. then her friends went off on me about it. im sittin there like "what did i do?" Can someone tell me from their perspective?