「」。
She told her older brother, 'I'm gonna tell on you!'
Literal
She [topic-は] older-brother [to-に] 'will-end-up-telling-on [reason/warning-から]' [quotative-と] said.
Classic childhood Japanese — the kind of line a younger sister hurls at her brother during a squabble. 言いつける ('to tattle on, tell on someone to authority') is the verb for snitching to a parent or teacher. The casual ~ちゃう (contraction of ~てしまう) carries a 'I'll definitely do it / it'll be done before you know it' flavor, and the trailing から sentence-finally hangs the threat in the air without finishing the consequence — the listener fills in 'so you'd better behave.' Subject omission is total: the speaker doesn't say who'll be told on or who'll do the telling, because the situation makes both obvious.