why does my sister do this

You can try to ask her not to sit on your lap. Tell her you don't like it when she sits on your lap. If your sister is attractive then it's understandable that, that happens. Not that this happens for everyone, but it happens for some. At your age it's also normal to have your hormones rushing. You're not a bad person, it's not happening on purpose. I found something on google that you could read: http://isitnormal.com/story/am-i-evil-my-little-sister-gives-me-a-boner-when-she-sits-on-my-lap-143815/

@fahrenheit451No no more sexual jetures okay

Thx

Yes shes quite pretty and thsnk you for the advise

Yeah i just dont wanna tell her to stop and make this strange and akward, yeah i suppse she probly knows

Does she frequently sits on your lap? Or perhaps she did do this when you're still young?

Yes frequently its not just in publicYeah she started quite a while ago

No you are not a bad person, and about your sister well there could be a couple of explanation why she does that but it's best to stop her like by saying you are heavy so get up it hurts or etc, just say it cool....

It's not weird for you to get erections. If you don't want to straight up ask her, just avoid sitting down when she could easily sit on your lap, or when you notice that she's going to sit, stand up and do something else. That way she'll get the hint and you wont have to go through the awkward part.