Quoting Unique Mama:" Oh fuc k that. She needs to give the tickets back OR the money. If she doesnt, let her know you'll be ... [snip!] ... to because she wont want to get into all that but it should scare her into doing the right thing. What a massive smurf >.<"
look in the mirror... you'd call the cops on her? oh come on woman, its comp'd movie theater tickets.
OP try to contact the theater today and speak with a manager about last night. dont even mention MIL getting comp'd tix. they will definitely either refund your money or give you more tickets.
id be pissed, but i wouldnt let it ruin my night. i wouldnt even ask for the tickets. think of it as her christmas present! give her a card and write "hope you enjoyed the movie theater 7 times over!"