Didn't one of the articles say there is two-year difference, so her sister is either 19 or 15. But it also mentioned something if her sister "survived", Adelina would not have been born (a little confusing). I guess that means it is her older sister.
No she's only 15, her younger sister. Adelina posts pictures of her on instagram and VKontakte from time to time. Cute girl and they seem very close.