Family Ties/Call him out on his enjoyment of your big tits.
This is too good to pass up. You're so tired of being ignored by your husband and having to pass up the more than willing advances of other men. Even as recently as a few months ago this would've seemed ludicrous, but your recent fantasies about your youngest have caused some moral slippage. And besides, you always enjoy teasing Jacob a bit. This isn't much different.
"You like them?" you ask, softly but clearly.
"Wh-what?" Jacob responds. His eyes dart guiltily around, but you can see him stealing glances at you between glancing at random corners of the RV.
You cup your breasts, careful to keep the bikini top in place. "These," you reply matter-of-factly.
"No, you're my mom, that's gross!" he replies, but he doesn't sound convincing.
"Do you want to see them?" you ask, undoing the loosely bound strings, but holding the tiny garment in place.
"No!" Jacob replies tersely. You just smile at him as he avoids eye contact, but after a moment, he glances back. After a few more quick glances, he looks squarely at your breasts, then into your eyes. You haven't retracted your offer or fixed your top. "You... you're serious?" he asks quietly.
"Of course, sweetie," you say. "You wouldn't share your game with me, so I had to find something else we could do together. I couldn't help but notice you staring at my breasts, so..."
"I want to see them," Jacob says.
"You want to see them... what?" you reply jokingly.
"I want to see your boobs, please," Jacob says. Without a word, you pull the cups of your bikini top away, revealing your firm breasts in all their glory to your eldest son. He puts his game down leans in across the table a little. "Can... can I touch?" he asks looking up at you. He looks like a kid in a toystore - well, he sort of is, just with a different sort of toy. You nod, and in an instant his hands are grabbing your bountiful mounds. He's rough and unpracticed, but the attention feels incredible. He naturally starts touching your small pink nipples, tweaking and pinching experimentally. You yelp at a particularly hard pinch, but say nothing and let him continue on his own, sighing softly.
"Are you enjoying yourself?" you ask, smiling.
"So much it hurts," Jacob says. He isn't wholly exaggerating, you think. He must be incredibly hard right now. You really ought to help him out, but you're already running a huge risk. Any moment now one of the twins or Heather could walk in, and there'd be nowhere to hide.