Until she is satisfied.

"We can keep going," you reassure her as you resume your ministrations. "We are family and so we support each other. I don't want you to feel nervous about cumming around...or on...or in...me." She giggles and nods.
"In you too, you said?"
"Sure. If we are out and you are getting hard and need to get off, I will need to eat your cum. Or if you need a fuck and I'm the only one around, I can just take your load inside in case you need to get off three or four times. That's so you won't have to stop and pull out."
"I never thought about all that before. If you are with me, I don't need to worry about having an unmanageable erection in public."
"Right," you smile.
"But even while we are at home...will you still take care of it? Your hands are so much larger than mine. I have trouble handling myself..."
"If you like."
"And..." she leans over you, "we can..." her hands grab your hips, then slide back, "have sex?" She squeezes your butt.
"Sure."
"We should get some practice in now; I want to make sure I can slide in easily."
"I don't know how easily you expect to slide in. This is a rather mighty girlcock." You don't stop her from kneading your butt though.
"You like your sister's big cock," she says as she straightens herself, "so I know you will do whatever it takes to fit me into you."
"Well I have used big toys before, but nothing this big."
"Your toys didn't cum either. Did I cum enough?"
"Excessively."
"I held back. I think I can cum more," she blurts out. "A lot more! What if I fill you? It will make a mess!"
"I'm stretchy, don't worry. You cum all you need to, and if I look bloated as a result don't worry."
"That makes me feel better. I'm pretty wet, can I...?"
"Can you...?"
"Can I stick it up your butt now?"
You lay back and spread your legs. "If you want," you smile.
Cassie fits the head between your cheeks. It's really big! You start to loosen your muscles for her. You flex them and then loosen them, working yourself open, like you did with your toys. Cassie pulls on your hips and pushes with a constant pressure. Her fat leaky cockhead is insistent. Then you feel what you call "the point of no return" where your muscles are opened by the invader to however wide they need to be. It's the best feeling usually. Your robotic sister is no different. Her cock surges, stretching you open. Once begun, your muscles are powerless to stop anything from stretching you further. This is no different as you feel your butt welcoming this largest of guests. Then without further ado, she inadvertently slides in half a foot.
"Oh gosh!" Cassie exclaims as she cums. "I'm not holding back. You are getting it all!"
You feel it inside. It spurts in these huge liquid surges inside of you. She goes only as long as she did before, but the volume expelled had to be much more. Your butt is filled well by her orgasm. You feel very bloated but only look slightly thus.
"That felt good, to let it all go," Cassie says.
"Yeah, I can feel that."
"Let's keep going."
Cassie resumes burying her two-foot cock into your butt.
"You like it so far?"
"You sure do pack one monster cock sis. With nice big balls to match."
"Yeah, you like me just the way I am. You are the best brother ever!"
She starts cumming in you again, after only managing another six inches. Deeper inside the cock shoots, no less than the first. If you felt full the first time, this time you feel like a water balloon as your abdomen bulges out.
"Bro, I can see how much you love my cum! Wow, you are gonna look really bloated by the end!"
"Ugh, yeah, it looks that way..." You remember a note you read from Cassie's manual. It said all ****-sized models contained safety measures to ensure the owner was not seriously injured during use. You smile at your new sister as that remembered thought puts your mind at ease.
"Still with me?" Cassie waves her hand in front of your eyes.
"Just remembering something. You can't be anywhere near satisfied yet. Why did you stop?"
"You zoned out, I wanted to make sure I didn't put you in a coma or something."
Without further banter, she resumes stuffing the second half of her cock into you.