Family Ties/Use your peep hole and watch them fuck.
With only a slight positional change on your bed and the removal of the picture that hangs on the wall to conceal the peep hole you are in the perfect position to watch the porn quality fuck happening in the next room. Heather's bed in directly across from where the peep hole has been cut offering you the widest angle possible for your covert viewing pleasure.
Jacob is flat on his back with Heather bouncing slowly up and down on his cock completely naked, her large, swollen titties moving up and down with every deliberate thrust. You'd give any thing to be the one under her sexy body right now, feeling her pregnant cunny milk your cock while she feeds you her sensitive nipples. "Mmmm Jakey..., do it baby, fucking cum in me!" Heather grunts as Jake goes rigid under her, indicating the start of his orgasm.
As soon as he starts cumming Heather grabs her tits and starts squeezing and molding them against the palms of her hands, using her finger tips to roughly twist her nipples causing her entire body to quiver. They cum in unison, shaking for a full thirty seconds to get before Heather collapses and slips down beside Jake and the two of them start cuddling. After about 5 minutes of pillow talk Jake gets up and starts getting dressed. Heather rolls onto he side, ideally toying with a nipple as she watch you brother prepare to leave.
"I wish you didn't have to go..., couldn't you just mow Ms. Jones' lawn some other time and spend the rest of the day plowing my pussy?" She teases as Jake finishes tying his shoes.
"I won't be gone more then a couple of hours." He says, disappointment evident on Heather's beautiful face.
"I know..." Heather says as she sits up and slips on over sided night shirt on. "This pregnant belly you gave me just makes me so fucking horny... I want cock all the time." She pouts.
Jake walks over to her and gives her a long deep kiss, "I'll be back soon baby..., and I promise to fuck mommy really good."
"You better!" Heather says as your brother leaves her room and you hear him tromp down the stairs. You than watch as your sister stands up and starts to look at her self in the mirror. She pulls the fabric of her night shirt taught against her belly, rubbing it lovingly as she let out a horny sigh when her hand ends up between her thighs. As horny as Heather obviously is right now and Jake out of the house for a few hours you figure this is your best shot at getting into her pussy. So you quietly slip out of you room and sneak down the hallway and go into Heather room closing the door behind you.
She hears the door close and immediately gives a giggle as she starts removing her night shirt assuming it's Jake that had re-entered the room. "I knew you wouldn't let your horny sister go wi...." Heather had gotten her shirt half way up her body before she realized it was you and not Jake that had walked in. You were staring right at her ass and pussy through the mirror when she dropped the night shirt and abruptly turned around. "Timmy what are you doing here?"
"I wanted to talk to you but I think you were expecting some one else." You comment evenly
"Oh that..., I was just.., I mean..., I..." Your sister cheek flush with a deep red blush, knowing how this must appear to her youngest brother.
"I know what you doing.., and who you were doing it for." You say, doing your best to hid the smile on your lips. "I know it was Jake that knocked you up."
Heather doesn't know how to respond so it would appear that you have the floor:
You could reassure her and tell about you and your mother.
Or you could trade keeping her and Jake's secret for a quick roll in the sack.