I only dated my ex-girlfriend for three months during the summer of 2009. Her sister recently moved back to the state I live in.
Her sister found me on a dating website and we started contacting each other on there, and now we text each other and I even call her sometimes on the phone.
If I was in your shoes OP, then it would depend on how the ex-GF and I broke up.
If we separated amicably and are still on good terms and truly over each other, then maybe it's OK to date the sister (although it's a good idea for the sister to discuss it with the ex).
Her sister asked me last night if I was intimate with my ex-girlfriend, and I said yes.