You help her.

"Oh, ok," you say, surprised she is so forward when just a short while ago she was so prude. "I can help you with this, but I still need to get you clean."
"Oh don't worry about that for now. It will dry and be all sticky, but I know you won't mind spending the extra time to clean it up," she tells you with a smile. She puts her hands on her hips and pushes her dick forward until it bumps your cheek. You relent and wrap your hands around her girth. She has lubricated you well, and you can slide your hands from head to base easily. After a few trips there and back, she asks, "Can you squeeze it harder as you go?" You squeeze her as hard as you can and give the length another pass. When you get back to the tip, you squeeze a drop of liquid from it which falls into your lap. You look up to see if that was to her liking, and the broader smile on her face gives you your answer. She bumps her dick against your chin as you look up. You had stopped when you looked up at her, but you resume your work quickly once her dick bumps against your lips. She pushes forward each time you approach the base of her dick, which causes it to bump your face and leave a wet spot. Each time you reach the tip, a drop of precum drips into your lap. This way, you get wetter the longer you go at it with her. Despite being so horny, she sure is taking a while to cum.
"What is taking so long?" you ask her.
"I'm a bit nervous about squirting, actually," she tells you. "It can get messy."
"So? Just let it fly and it will land where it lands," you state.
"Kay," is all you get before a wad jets into your mouth. The next hits your chin and splatters. The ropes fly too quickly and randomly to aim for. When she finally looks down, your sister sees she has covered a lot of your front with her cum.
"That was pretty amazing, and tasty," you tell her after you swallow what you caught in your mouth.
"Wow, that was really good!"
"Do you feel satisfied? You don't look it," you tell her eyeing her still-hard cock.
"I'm just excited, is all..." she trails off.