You smile at her "Well, you want a little brother?" and you close your eyes, willing reality to change with you, making yourself younger, switching from older teen, to the same age as Becca, and younger still, until you are 6 years old, smiling up at her. Your room changed as well, toys and clothes appropriate to your new age, and she gasps "OMG, you are so cute, you look just like a little brother I would love to have,but you're not really. It's still you inside" You nod "But I swapped reality, so even mom and dad think I am this old, all my schools and friends, everything is swapped so I am this old, like a 6 year old"
Becca blinks "Well, not everything, obviously you still have your older brother mind, not like you are actually acting like a 6 year old." she smiles and holds out her hands to hold yours "Thank you for doing this for me John, I love you, little brother, hehe" and you take her hands, your tiny hands in hers, smiling.
The smile turns to shock when she uses the moment to slip the ring off your finger, holding it and smiling a little more mischievously. "Becca, no!" but she does, slipping it on, and looking at you "I love you little brother"