Welcome to kittenhood!!! Honestly, sounds quite normal to me, and she will most likely grow out of it. How old is this girl? How long have you had her? (and how is it possible that she has a half sister from a different litter that is only 1 or 2 months older?).
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler