Family Ties/Have Jacob make Jessie a bit more "comfortable"
"Jakey, I think you should help your little sister get a bit more comfortable," you say, grabbing at one of your tits.
He stares back at you blankly. "Umm... what?"
"Use your imagination," you offer.
Jacob looks at Jessie, the girl looking up at her bigger brother with an honest smile. He shuffles forward until they are only a foot apart, his dick bobbing around in front of her which she stares at with wonder. He leans in and reaches behind her, fiddling with the strings of Jess's bikini top. The tip of his dick pokes into her stomach more than once, bringing her to stifle her giggles while he fumbles away.
With Jacob relieving his sister of her top you relieve him of his bottoms, wiggling his boxers and shorts from his ankles. You look up just as the top is pulled away from your daughter's chest. Jacob sets it down on the table with one hand while absentmindedly squeezing his dick with the other.
Now that your daughter matches you she does indeed seem a bit more comfortable, even excited. Jacob has a death grip on his cock, either trying to hide it from view or strangle it into submission, but he still gladly and openly roams his eyes over both you and his sister's bodies.