Is it normal my sister sits on my lap

Totally not normal for your own sister to sit on your lap like that! Eat more beans and start farting as much as you can when she's near you!

Tell her to stop.
It's making you uncomfortable and she has no right to force herself onto you. Maybe that would be more obvious if the sexes were reversed.

im a lady and this is pretty normal finding out about sex let her make you get a hard on let her feel it no issues there ,and yes as other post above says feel her boobs a little ,ok then go wank in private ok then you will be having a sexual experience and all good ,maybe if she keeps it up ,my older brother taught me to masturbate my self when i was 12 this was all good and nothing to be ashamed about ,there are many things that can be done with out sex ,but if it comes to sex please do not ever cum in her ok best to just play ,maybe buy her a vibrator if you have money if not do the best a brother can do

Take the chance. She is inviting you to eat her. Just press ur erect cock in her ass crack. She needs a cock. Do not let her go and give a chance to other person to suck her tasty pussy and ass. Slowly try to touch her boobs when she sits on u. Give ur cock in her mouth and give some slow stroke. Tell her to lick ur balls.

How old are you both ?
She says she is trying to get comfortable meaning that she is trying to grind your boner cock in to her ass to get her to cum and she is giving you hints that she wants you to fuck her in her dirty ass.
Then once you do that , she will manipulate your cock in to her bald cunt and then in to her mouth.
Next time she sits on your lap , grab a handful of tit and then stick your hand up her loose thin shorts and finger her bald twat and slide a finger in her poop chute.
Has she ever seen your naked boner , either walking around the house naked or changing your clothes or in the shower when she comes in to go to the bathroom and she sees you through the shower curtain or has she ever caught you jerking off and ask you if you need any help and did you say sure and did she help you jack off ?
Has she ever seen any other male's naked boner in person ?
Did you take baths together when you were kids and soap up each other and wash each other off ?

Try to fuck that ass

Let her keep doing it, but..Use your hands on her. Grab her ass, go inside, finger her..Sounds like she's more than inviting you to explore. Also, depending on where her hands are, let her feel your hard cock for herself. This is a huge opportunity for FUN..

Ask your sister if she likes it. If she says yes then keep doing it.