That's right, X17online caught 15-year-old Ali Lohan trying to sneak out the back door of Crown Bar after partying with her big sister/bad influence Lindsay all night.

In the first video, Lindsay enters the back of the bar with some friends, leaving Ali behind. But Ali somehow snuck in just 10 minutes later, and stayed with her big sis in the club until right before closing, when we spotted Ali running out to the waiting SUV (second vid)!

Says a witness at the club:

    They stayed at the club for an hour. They were with a couple of friends. They were both happy and bubbly. Ali drank water and Lindsay drank red bull. They hung out at their table while dancing to the beats....I guess Samantha wasn't invited. They left at 1am.

We all know Hollywood starts 'em young, but 15? Seriously?? Where is her mother???