Blaming your little sister for the broken thing gets you out of trouble and into her memory forever.

Getting out of trouble by pointing at someone smaller works nearly every time and costs more than it appears to. You keep the afternoon. She keeps the memory of being disbelieved by the adults while the one person who knew the truth stood there saying nothing. Children are precise about this, and the record outlasts whatever the punishment would have been.

If it has already happened, go back and put it right, even late, in front of the same people who heard it. Tell her before you tell them. If it is happening now, take the smaller loss, because the whole cost is one uncomfortable conversation.