You spend some more time cuddling with Sarah, then took the shower and head to your room as you have a homework to do, while your sister do same. A few hours later you heard as the front door opens, it's mom or Crystal returns you assume, also you heard Sarah's voice. You quickly finish your homework and head to see what's going on. In the living room you saw your family, mom and Crystal is sitting on the sofa while Sarah is say something with a happy smile, they turned to you as you enter the room.
"Oh, there you are, my boy! Sarah told us you two have sex, is this right?"
"Well, yeah..."
"Heh, never thought he'll do it with sister!" - said Crystal with a grin.
"Oh, I'm so proud of you!" - your mom sweep a fake tear. - "My little boy is grown up."
"Um, mom, can you stop, it's too embarrassing!"
"He-he, sorry if this too much, but messing with you is so fun, and I think we need to celebrate this! I even bought a cake!"
"Yay! Cake!" - as soon as your little sister heard about cake she dart into kitchen.
"Wait, you really bought a cake for this?"
"No, not specially for this. I didn't told you yet, but I've got a raise!" - Lisa said proudly. - "Crystal, can you go and save the cake from your sister?"
"Okay. By the way, it mean that now I can fuck Doug?" - asks your big sister before leaving the room.
"Well, as long as he doesn't mind!"
Crystal winked to you and left the room, so now you're with your mother alone, what can happen next?