Get the other girls involved.

"You three hotties- why don't you stop playing with each other...and start playing with Lexi here," you suggest.

All three girls smile widely and nod at your suggestion. They all crawl out of the hot tub and approach you and Lexi. You admire their naked bodies once again, which helps keep your member hard as a rock.

"I don't know if I'm into girls," Lexi says.
"You don't have to be- they just have to be into you," you explain.
"Alright. If you can top that last performance of yours, I'll agree to anything!"
"I'll hold you to that. Now let's change positions."

You pull out of your stepsister and help her sit up.

"What did you have in mind?"
"I want you to ride me," you answer, "It leaves that hot body of yours wide-open for these three and it lets you control just how hard you get fucked."
"How could I say 'no' to that?!?"

You sit back down on the laid-out chair, propping the back up a bit so that you're still leaning up a bit. Lexi stares down at your rock-hard dick and over at her friends. They smile and 'invite her' to sit on your dick. You do the same.

She stands over the chair and slowly lowers herself onto your cock. She grunts as you fill her up once again. She eventually 'hits bottom' and rotates her hips.

"Ladies- she's ready for you," you say, gesturing for them to come over.
"What did you have in mind," Diddy asks.
"Well, you and Melanie did such a good job on Justine's breasts...so why don't you give Lexi the same treatment?"
"O-kay," the two girls say in unison, moving into position.
"What about me," Justine asks.
"Well, her lips are free," you reply, "That or you could always rub those cute little pussy lips of hers."

Justine smiles, but says nothing. You see her move out of sight as you focus on Lexi.

Your step-sister begins to move her body up and down, fucking you to her liking. As she gets her rhythm going, you hear her moans increase. You see that Diddy and Melanie have each latched onto a breast and are beginning to suck. They manage to keep sucking as Lexi fucks you.

You hear an even higher-pitched moan, which is quickly muffled. You see that Justine has taken both of your suggestions. With one hand, she is rubbing on Lexi's clit and she is making out with her as well.

After a few minutes of this, you can feel Lexi's pussy tightening around your dick. You know what you have to do.

"Justine- free up her mouth for a minute," you say.

The big-breasted beauty breaks her lip lock with Lexi, allowing her to emote what she's going through.

"Fuck- I'm cumming so hard," she exclaims, "This is the big o-one!!!"

Justine kisses her all over her face and neck while Lexi cums. Diddy and Melanie, in turn, do the same with her breasts.

It takes an enormous amount of self-control, but you manage to hold out and not cum. You can feel Lexi's pussy trying to milk you...but to no avail. It will be even better when it comes later.

As her orgasm concludes, Lexi leans backwards, her legs staying in place to hold your dick in her.

"Fuck- that was good," Lexi says, "That was even better than the last one. How can I make it up to you- all four of you?"