[tex]▪▪▪▪▪▪▪▪▪▪▪▪▪ {\huge\mathfrak{Answer}}▪▪▪▪▪▪▪▪▪▪▪▪▪▪[/tex]
The Age of your sister at that time will be ~
[tex] \boxed{12 \: \: years \: \: old}[/tex]
[tex] \large \boxed{ \mathfrak{Step\:\: By\:\:Step\:\:Explanation}}[/tex]
The number of year passed till you get 15 is ~
So, at the same time your sister's age will be ~