On her tits of course...

You struggle to contain yourself as you wonder where to spray your load... In the end, the temptation of cumming in her large tits wins out... Despite the risks, you point your cock right at her exposed chest and shoot more than a few strings of thick white jizz onto her breasts...

The sight of your cum on your bitchy sister makes you laugh inside as you put your dick away... You decide to leave the mess, knowing that she'll likely wake up confused but unable to prove anything had happened... At least you hope so...