ranka is too selfish. with alto and others being held as hostages, along with sheryl being ill, all she can think about is her own selfish thoughts about not being able to give her bitter cookies to alto, rather than the well being of everyone. She flies across to another world, probably in secret or against Ozma's wishes, for her own benefit, so she can say "happy birthday, alto kun." she isn't considering anyone else's feelings but her own.
Sheryl is just like Misa. Prideful, confident on the outside, but I suspect Sheryl is insecure inside. She is also capable of accepting ranka being with Alto if that is who he chooses, sorta like Misa. Sheryl gives Alto what Alto wants, where as ranka gives alto what ranka wants.
even SDF1 couldn't stop me from giving this episode my first neg.