A bit of fan-fiction (from my twisted mind), just for fun ...
[cut to the Beth Scene, where she is comforting Morty, at the end of the Planetina story]
Beth: Would it help if Mommy made it feel better here?
Morty: Really? W-what about Morticia?
Beth: It's okay, she's sleeping in the Summers' room tonight. Actually, she was oddly excited about it.
[cut to the Summers' room]
The two Summers are in bed together, chatting, laughing, and being a bit flirtatious with each other
On the other side of the room, Morticia is sitting against the wall, that separates that room from Morty's.
She is gently pressing her ear against the wall, trying to listen to what's going on in Morty's room.
Summer: Hey Morticia, I told you, you don't need to sleep on the floor, you can sleep in the bed with us.
As she is saying it, Summer suddenly realises that Morticia is eavesdropping.
Summer: Hey you little perv, are you listening to our brother masturbating, again?
Morticia: E-erm (Moticia blushes)
Wild Summer: God, she really is a little perv.
Morticia: I'm not listening to him masturbating. Beth is in there with him.
Both Summers stop grinning and look shocked.
Summer: Oh my god, is he fucking her again?
Morticia: Not yet, but it sounds like he might be just about to. Do you wanna watch?
Summer: No way, I'm not getting caught, peeping through the door on my Mom and brother having sex.
Wild Summer bites her lip, as she thinks about it.
Morticia: we don't have to peep through the door, Rick invented a one way viewing portal, that allows him to see into other dimensions, but it doesn't show up on the other side, and it works on walls too.
Summer looks stunned and doesn't reply.
Wild Summer bites her lip and softly says
Wild Summer: Fuck yea ... erm, I mean, Dr Wong said it would be good for me to watch someone else have sex.
Morticia pulls out an odd looking portal gun and points it at the wall. The wall suddenly becomes see through. They can see Beth and Morty sitting on the bed, with his head deep between Beth's exposed breasts, while Beth starts stroking his cock.
You can use your imagination as to what happens next, or if you ask me really nicely, I'll maybe continue the story later.