Family Ties/Tell her what your twin is up to.
Hoping to drive a wedge permanently between your dad and your mom, while at the same time leading her deeper into your arms you decide to tell her what you had witnessed your dad doing to your sister right before mounting her. "Mommy..," You start out in a whisper, "When I was walking back from the bathroom I saw Jessie straddling dad's lap as he was driving. She had her jean shorts off and he was tickling her and grabbing her ass..., tugging at her bikini top." Admittedly that's a slight exaggeration to what you actually saw but it should suit your purposes just fine.
"I know baby..., Sweetheart there is something I need to tell you." She says returning your whisper as she rolls onto her side, taking you with her and effectively pinning you between her and the back of the make shift couch. You're more than a little shocked at her confession and equally interested in what she still needs to reveal. With your head loosely nestled between her breasts you are still able to make eye contact. Your mother's toned stomach is pressed tightly against your quickly hardening cock as she starts to speak again. "A few days ago while you were at school your dad came home early to find me...., well, playing with myself. Now, that's not such a big deal, what made him so mad is that I was making myself cum on your bed."
You can't help the look of confusion that crosses your face, hindered by the fact she is now grinding her abdomen against your prick causing you to shiver. "The fight was pretty bad and when all was said and done I told him how badly I wanted you inside me. He than admitted to me that he was hot for Jessie so we struck a deal: Your cock for her pussy. We agreed that neither one of us would force either one of you to do something you don't want to do but that this weekend would be about getting you between my thighs and your dad getting between Jessie's."
You can hardly believe what's coming out of your mom's mouth. The idea that she has been playing with her pussy while lying on your bed drives you crazy. A hundred questions run through your mind all at once and try to make it out of your mouth at the same time. "So is dad going to fuck Jessie? What about Heather and Jake? Does he know you and I are already fooling around?" You ask in a heated rush, still having to resist the urge to take your mother's nipple into your mouth and start sucking on it.
She lets out a throaty giggle that turns into a moan as the continues to grind her tummy into your horny, preteen cock. Caressing the back of your head and pulling you deeper into her breasts she does her best to answer your questions. "Jake has been horny for your big sister for a long time now. He even made a fake online ID so he could try to seduce her. She caught him last night and is in the back room with him right now talking it over. I wouldn't be a bit surprised if they are already fucking." She says as she shifts her position again to lay you on your back so she can straddle your hips, rubbing her pussy against rock hard bone. "As far as your sister and your dad, let them worry about them." She suggests, grinding herself hard into your cock. "This weekend is about you and me. You enjoying what my body has to offer. My warm lips....." She says, leaning down and kissing you briefly, but passionately. "My tight, wet pussy...." She says as she grabs you cock and impales herself on it making her whole body shake. "And these big fucking knockers are all yours..." She moans, leaning over and bracing her self on the arm of the couch so her titties are hanging directly over your face.
As her pussy clamps hungrily around your cock and she slowly starts ride your rocket you decide your are done with talking and start sucking her left nipple as hard as you can. The two of you make love for the rest of the ride to the lake, ending with dousing her fertile womb with your seed about 15 minutes away from your family's usual spot. This gives you just enough time get cleaned up and dressed before the RV is parked.
Shortly after parking Jessie stumbles into the center room with a silly smile on her face, adjusting her shorts that she obviously just put back on. She is followed by your dad who tells the two of you that Jessie and him are heading out for a stroll along the lakes shore. You and your sister share a glance before they walk out the door just as you are joined by Heather and Jake, making a similar excuse to be alone together. After they leave your mother drops to her knees in front of you and grabs you up in her arms, kissing you more deeply then you had ever seen her kiss your dad. At the conclusion of that ground shaking kiss she asks, "See it's just you and me. So what would my little lover like to do."
You could suggest: