Based on the explicit "Siblings Game" comics.
"Okay, I just changed your little sister's diaper, and please take care of this house while we're gone. And make sure your brother doesn't do anything illegal, like burning the house down. Don't worry, Mom. Me, Bart, and Maggie will be fine, and you can count on me. Good. We'll be back by 10:00 PM this evening. See you."
After the parents left the house, Lisa would walk over to Maggie, who started to feel sleepy. "Alright, let's get you to bed, baby sister. (suck!)" The two sisters then to the nursery, where Lisa would lay Maggie down in her crib. "Sweet dreams, Maggie." she said quietly, watching Maggie sleep like a baby while sucking her thumb.
After gently putting Maggie to sleep, Lisa's big brother, Bart, walked over to her and was impressed with her parenting skills. "Wow, Lis'. It looks like you'll be a great mother one time. Really? Yep, because all three of us may be siblings, but when we're together, it's really cute and sweet."
Hearing him say that, the two siblings held hands and went to Lisa's room. There, she closed her bedroom door. "Now, with Mom and Dad gone for the evening, and Maggie sleeping like a baby, we siblings are alone for now. Watch this." Lisa would perform a cute act in front of her brother, which delighted him a lot. "Yeah, baby. Aw, you're too cute and adorable. Thanks. Did you spot my underwear while I was performing that act in front of you?" she asked. "I don't think so. That's strange, because you always spot my panties when I perform acts like that. Aw, come here, my sweet little sister." Hearing him loud and clear, the little girl would come closer to her brother, as he would get her to lay down on his lap. "First order of business is to get these shoes off so your feet can breathe. You mind if I lift your orange skirt to reveal your tummy? Nah. Go ahead."
Bart would lift up his sister's skirt to reveal her tummy, as well as her underwear. "Aw, Bart. Now you've made me look so much cuter by not only showing my tummy, but my undies as well. Because even though you're 8 years old, and I don't care how old we are, you'll always be my sweet little sister." both said sweetly.
As the two siblings embraced in their cute moment, the little man would land his hand on her tummy, slowly rubbing it, followed by landing his index finger on her bellybutton. "Oh, yeah. Rubbing my tummy and feeling my bellybutton as always, when it's nobody else in either my room or your room, with the exception of the two of us. I remember when the both of us were alone in your room and without clothing, and after we gave ourselves a taste of each other's pee-pees, we took care of each other as if we were babies. And as I said before, you are the cutest and sweetest sister I could ever have, along with Maggie. Would you like to feel me caressing your upper legs? Yes, because that would feel nice."
The little man would then feel the warmth of his sister's legs by softly caressing them. "Doesn't it feel so warm and soft? It sure does, so keep it going, baby. Um, you're feeling my pee-pee a little. Oh, sorry. That's okay. Yep, until Mom and Dad come back home this evening, you and Maggie will be fine with me while the three of us are alone."
Then after spending sibling time with each other again, the little man would help his little sister get onto her bed so she could dose off for a little bit. "I bet you'll be a little restful before dressing in your sleeping clothes. Yeah. Sweet dreams."
(Maggie whimpering)
(hushed) "Sounds like my sweet baby sister needs me again. Coming, Maggie! Your big brother will be right there."