Is it normal for my stepsister to sit on my lap?

My stepsister and I have been living together for 6 years. My mom married her dad when we were 12, and now we're both 18. My stepsister both know eachother very well.

So one day, when we were 13, during the car ride back to home from Disney world, my mom was sleepy so she went on the back seat to sleep, and my stepsister chose to sit on my lap instead of go upfront for the rest of the ride. At first it was comfortable but then it got akward. I immediately got hard and asked her to lift up for a sec and saw she was wearing a light pink thong and my penis was exactly under it (covered with my shorts of course she didn't mind because I've seen it before, but now it just seemed.. better. I didn't control my tempatation and I started to place my hand under her skirt and I felt her ass and stroked it for 40 seconds or so. She noticed and smiled, but then she looked very startled. I was worried she was going to get off my lap and go to the front seat, so I stopped. My worrying changed to pleasurement as she leaned back against my chest and gave me a quick kiss on the lips, and I rolled off my pants and just calmy rubbed around. Very quietly, so no one would notice. We stopped later because we didn't want to be noticed by dad, so I just placed my fingers around.

there were many times similar stuff happened, even alone at home with her sitting on my lap or naked in the shower, but I wouldn't describe those. nothing important happened except one time where she sat and rubbed against my face and my penis on the couch in yoga pants, and the other time when we were playing video games, alone in the home,(I wore nike shorts, no shirt, she wore bikini with a g-string) and she sat down on the floor with her butt up. It was sorta doggy style except she was completely laying down and I was sitting down on her legs (she didn't mind) normally except with my legs apart, so my penis could feel her butt. That's what we did every time we played video games and our parents weren't home, until...

We did that a lot until we went to an aquatic park a couple weeks earlier and something similar to the car-scenerio. Except she was now fully wearing her dark blue thong w/ g-string and matching bra. I wore a small pair of basketball shorts since I didn't have a real swimming outfit, and swam shirtless. We were both sorta wet from the swimming and I was sitting down, admiring her swim. I got embarrassed because she was so pretty and I got a random boner. She got up out of the pool and sat down next to me, I'm pretty sure she noticed my boner. She smiled and brushed her silky hair infront of me, got up and hugged me with a kiss on my lips and me holding her hips and then sat between my lap again. The slightest rub made my penis throb and she noticed. She rubbed back and fourth and I kissed her more and almost went overboard. But then, inevitably cum poured out and, even though I REALLY wanted to do more and she did too, I got her off and darted to the restroom. She talked to me and we reluctantly agreed to stop her from sitting on my lap unless we had to, which was something we both hated to agree on.

The thing is, my stepsister loves to sit on my lap and I love her sitting on it, especially when she wears those thongs and stuff. I don't have a massive sexual desire to her, but I randomly came pretty much inside her ass and I do but don't want something like that again. We both don't mind random boners because it's funny and I guess she likes it (I do too) but cumming inside of her is a whole different story. I really, really like her and will do it if it's ok, but is it ok for her to sit on my lap like that? Do we need to put limitations to no bikinis or stuff? and if I cum and one thing turns to another is it ok to have.. yeah. I wouldn't mind because she's really hot and we're not genetically related so that would be interesting.