"Come on Nick," she cooed, "Come into my breast." Her hands held one of her big breasts and opened the nipple revealing a flesh-like tunnel. You blinked a little chilled how a hole like that existed now. This could turn into a very weird fetish. You decided to hold back a little and possibly try and reverse this.
"Um...We're brother and sister right?"
She looked at you confused. "Yes of course we are! What do you mean? Am I your girlfriend too?"
You stammered going blank as you realized just how cute and hot she is. "Yes! I mean no...wait a sec..."
She giggled and then you realized your lame mistake. Fuck, you thought.
"It's okay my little brother. Rosie can be your big sis and your girlfriend! I'm here for you aren't I?"

You sighed. This may not be a bad change, besides, you knew that you could probably change her back by stating what she is to you.
"So are you coming into my beautiful breasts?"

You shrugged. "Hell, why not..." you muttered and then suddenly flames shot out of her breasts!!
"No hell! I'll go into your breasts...as long as I can breathe and it's not burning in there," you said. She nodded giggling. No more flames appeared as the tunnel got wider.
You slowly started slide in with your head first. It was apparent that your sister was getting impatient as she gave a little "kick" to you that made you slide further in. She let go of her breasts and moaned very load.
"Oh god! Nick! Nick! I love you!!"
You gasped as your whole body was inside. On the outside, one of breasts was huge with you inside of her. It looked odd due to the difference in size of her breasts. That was going to change soon.
"Oh my god...I'm cumming!!" she yelled as came onto her bed side. The let loose and fell to her bed, tired.
"Oohh...Nick...I'm going to rest now. Since you wanted to be in my 'breasts', It should be able to make the size even..." She fell asleep just like that after taking her brother in like that. You weren't sure what she was mumbling about but you clearly heard the plural form of 'breast'. You were a little scared since you weren't sure what was going to happen to yourself.

Suddenly the walls of her breasts compressed and leaked out a milky substance. You slowly slided deeper and deeper without any knowledge of where you were going. It was so dark and only thing you head was the walls compressing back and forth. You couldn't help but slowly started to drift away yourself. You weren't sure if you were awake or not since it was dark anyway.