My sister is wearing my girlfriends clothes?

In two ocions my girlfriend forgot a coat and a sweater in my house. when she asked for them i couldnt find it, so she tought she just misplaced it at her house and went insane looking for it for like a month (expensive coat) We asked my sister if she new anything about it and she said no and was helping us look for it. its been 2 months now and I just found out my sister had the coat all this time. what should i do , my girldfriend is going to kick her *** and i dont want my family to get mad at her.