you + 1 sister (older than you); you might say:
My elder/older sister got married.
you + 2 sisters (both older than you); you could say:
My 'older sister got married. (' means more emphasis; talking about sister 2, older than your other sister)
One of my elder/older sisters got married. (talking about either one of your sisters)
My eldest/oldest sister got married. (talking about sister 2)
In case of more than two sisters older than you, it's really down to you to express what you want to in a way that is not too much ambiguous to your listener.