Question about sisterly behavior

Okay, a work in progress wasn't supposed to become erotica, but it appears to be headed deep into that territory and I need some advice.

Our main character has a younger sister who is basically a slut. She's about to graduate high school and has given herself to boys in order to get her homework done so she can pass and still have a good time. Our main character has, through circumstances, ended up at a quarry where skinny dippers hang out and it turns out her younger sister is there. Suffice it to say, our main character and her boyfriend consequently save her younger sister from embarrassment at the very least, but now the younger sister has seen the boyfriend and is making a play for him. Our main character works as a waitress and is a few years older, she's not ignorant of the facts of life, but she doesn't hop from bed to bed like her younger sister.

The boyfriend is falling in love with our main character, but she's unaware of that as yet. However, on a subsequent trip to the quarry because she knows her younger sister too well and that the younger sister will attempt to get even with the individual who almost caused her to be embarrassed and possibly worse, after seeing that her younger sister isn't in any trouble though they arrived too late to intervene as they'd planned, the main character decides to take her boyfriend behind some boulders just to stake her claim, so to speak. Is this reasonable even though the younger sister finds them a few minutes later doing the deed?

Any suggestions on how far sisters will behave in each other's presence?

I'm going on the premise that main character sister didn't miss seeing her younger sister make an overt play on the boyfriend and even openly attempt to feel him up (remember they're at a place where everyone is skinny dipping) and concludes that the only way to fight her younger sister is with fire. Of course, the part about the younger sister happening upon them behind the boulders wasn't planned by the main character, but she's determined she's not going to be embarrassed at being on the bottom or halt her activity because that would make her sister eventually win. Logical in an emotional way? I'm not certain.

However, this builds new feelings in the main character when she realizes that she also likes what she experienced for the first time plus she's more determined that she won't let her younger sister push her aside. I recognize that all sisters aren't like this, but not all sisters have one about to mutate(?) into a fairy complete with wings.