u have two older sisters.
u are C and they are A and B.
A is older than B.
here it is:
A is your 大姐(dajie)
B is your 二姐(erjie)
if u have more, u can continue: 三姐(sanjie) 四姐(sijie).............
if they are brothers: 大哥(dage) 二哥(erge) 三哥(sange) ..............
very simple
:D