I tried some random truth tables, then I wrote it on paper & got:

====can you truthfully say that you are the middle sister

-my shorthand:  l=not()
1:t n
3:l y                not(are-u-mid=n      & are-u-tt=n)
2:m if t:y  l:y_as   not(are-u-mid=not(y) & are-u-tt=n)

So marry the one that says no



Longer version: 
the eldest tells the truth that she is truthfully not the middle sister, w/a no.
the youngest lies about truthfully saying that she is the middle sister, w/a yes.
the middle sister, when acting like the truth telling   has to say yes
               and when acting like the young/liar also has to say yes, so w/a yes.