How old are you now?
Penetration can't ever happen through clothing. Clothing has to be removed or unzipped or opened in some way. I also assure you that a female can't be penetrated without reacting to it. If your niece was a young child, it would have hurt her, and wouldn't be like an adult relationship where things are much easier. It wouldn't just slip in her.
She'd have reacted, and everyone in the car would have noticed.
Hi, tryingtolearn - NO, there is absolutely NO chance there was any penetration, so you can put that completely out of your mind - no issue whatsoever with that. Therefore, there's no possibility of actual sexual 'contact', pregnancy, or transmission of STIs, so just relax!
I sense that you are feeling some guilt about what happened, but as I'm sure you know, at age 15, guys get erections all the time - even with no sexual stimulation whatsoever! All of us remember times getting them in class for no reason at all & having to hold our books in front of us on the way to the next class! The added stimulation - even though it was unintended - of having a young girl sitting in your lap, obviously putting some pressure/friction on your genitals, would just add fuel to the fire, so to speak! And of course, we know the tendency of young kids to move around, squirm, etc., would only make the problem worse. Then you have the motion of the car going over bumps in the road, vibrations, etc. - it wouldn't even have been surprising to ejaculate under those conditions. If I put myself in your place, I guarantee you the same thing would've happened to me at that age, even if I had absolutely NO sexual thoughts at the time...
So relax - it was an entirely normal reaction for a guy at 15, which is right at the peak of hormones & frequent unwanted erections. In fact, I think it's odd that an adult would even suggest having a young girl sit in a 15 yr-old guy's lap, knowing the potential for that to happen. Whoever proposed that should have suggested the girl sit on one of her older sister's or female cousin's lap, or maybe her Mom's or Aunt's lap, NOT a 15 yr-old GUY's lap!