I would stay away, and be really careful how you advise your gf of the situation..we as men may make hyperbolic assumptions about women's general pettiness and jealousies, but I don't think any of us really comprehend the hurtful, damn near evil competitive streak that can exist amongst sisters...She may not be into you at all really, someone else euphemised she may see you as a safe target, but honestly I think she sees you as something she can hold over her sister, who if im gathering correctly the situation, the older sister might see your presence as the younger flaunting her relationship success, with a guy the older sister's age to boot.
Id ask your gf about her sister's exes and see if you fit the trend; might be nothing but yet another chess piece in their game of life.