You take action.

Relieved, you resolve to try to hurry things along. The next day, you head to the store early and buy a few things. When you get back she is wearing her spandex outfit again and is milling about the place.

"Hey, I got you some things while I was out." Cassie heads over and starts pulling things from the bags. She pulls out a few shirts and shorts and then produces a bikini. She holds it up so you can see.

"A bikini? You're my brother! This is sooo embarassing!"

"Well, it's summer! You're a girl, you need girly swimwear! Unless you want to just wear trunks like me and show off your tits to everyone!" She calms down a bit.

"Well, thanks for the thought," she mutters. "I can't fit into them anyways."

"Nonsense, I want you to try everything on and show me it fits, so that I can take back what doesn't," you order her. She opens her mouth to protest but you cut her off. "Unless you want to keep with that number you have on now forever. And start with that since you are so sure that it won't fit."

"But...well, you'll see..." Cassie trails off then goes to her room with the bags. She comes out only a minute later and comes to a stop in front of you. Her balls nestle in the thong bottoms, but her cock just dangles there. Her nipples are visible under the thin material of the top.
"Happy about seeing your younger sister in a bikini?" she asks with a mix of anger and disgust. You gesture for her to turn and she does. The thong disappears inside of her crack as you had hoped; so you basically look at her naked ass. She finishes turning and stands there.

"Don't be so embarrassed. You have a gorgeous body," you tell her when she is facing you once again.
"Minus the huge turnoff of a two foot cock...what the fuck is that about?" she asks pointing at your cock which is once again hard and sticking out.

"Yea, sorry. You may be my sister, but you are really hot! I can't help it!"

"Cool, what a great day! My brother wants to bang his own sister!" She goes stomping off, her ass jiggling sexily.