If you didn't say anything at the time, you missed your opportunity. If he freaks out again, say something jokey like "thank goodness Godzilla saved half of Tokyo!" or "I guess its HIS cake now!"

My brother has the best behaved kids I know. At Easter I bought Bakery made cupcakes. His kids know they aren't allowed sweets, so when the adults were having a conversation in one room, they were in another licking the frosting off of $20 worth of cupcakes. I thought it was funny. Funny! They took a calculated risk (knew they weren't allowed sweets but weighed it against dad's reaction) and went for it. They're KIDS! At a PARTY. I was upset over the money wasted, but I was the idiot who bought gourmet cupcakes for a family gathering.... besides it was my brother's job to correct them. By the way, Cameron looked at me in shock because he knew he would never get away with something like that.... but his misbehavior is of a different variety -- he's high energy and generally embrasses us in public as strangers watch either in horror or laughing their tooshies off (people think he's much older because of how big he is. He's 5 but defintely looks 8. They tell me he should be acting his age.... yeah, have a five year old act like an eight year old and you can keep waiting three years to see it happen).

As for saying something to him or your sister. Tell your sister it hurts you. Use "I" statements (see? my time in therapy has some uses! ) and see if she can talk to him. If she disagrees, then agree to disagree and drop it. You told her how it makes you feel. Its her husband and her relationship and she can talk to him about his behavior. It will only cause animosity if you skipped over her and hent to him. I am still smarting 10 years later when my brother did that to my husband.... they were right, but they went about it all wrong.