She resists weakly.

"No, don't," she says weakly. She tries to pull away, but you hold her bikini strings as you finish undoing the knot. She keeps trying to pull away, but she just ends up rubbing her nice round ass against your leaking cock. You let go of the strings once you undo the knot. Her top falls to the ground as you grab her hips again and pull her into you so your cock nestles in between Cassie's buttcheeks.
"Fuck, bro, you are really making my ass wet," Cassie mewls. "I mean, ew," she corrects herself half-heartedly. You turn her slowly. Your dick slides out of her crack and leaves a wet trail around to her front. Her dick is fully erect and standing about two feet out from her. Her tip is dripping at a regular rate. It formed a small puddle on the floor where she was standing. You stare at it, then look up to see she was watching you look. You grab her shaft as you look at her. You walk to the end of it, and let the end sit at the top of your ass so all the fluid drips onto your butt. When you aim it right, the drips go right between your buttocks and flow downwards. After a few drips that way you let go of her shaft and wiggle your ass around to smear the drops in your crack to ensure your cheeks slide against each other.
"Ew, bro," she finally speaks up, "you got my clear stuff all over your butt. It looks like you got it in your crack too on purpose. Does my butt look all shiny like that?"
"Yes, and it looks sexier too! Why do you ask?"
"I get so horny when I think about lubing an ass...mine or yours! I don't understand why." She pulls her bottoms down. "I think I want to do nasty things to you! I want to cram my entire cock up your ass and shoot load after load of my cum into you! But that's nasty! You are my brother and you aren't even gay! So I can't...but I still want to!" She seems conflicted.
"Hey, being gay only means you are into other guys. Straight guys can like ass play too!"
"But, still, you are my family! That's ****!" She runs off to her room before you can grab her again.