I'm glad you like my site. I like how thoughtful and considerate you are of your ex-girlfriend's sister. More people should possess your level of partial identification for others.
You have a couple of different ways to handle the predicament. The way that you choose has to fit your personality. The key is to find a way of letting her know that you aren't interested in her romantically without slapping her in the face. I will give you some options and you can choose the one that feels right for you.
The first option is the indirect approach, in which you avoid her advances until she gets the hint. To do this, you would politely decline her requests to get together, saying that you are busy, and soon she will realize that you aren't interested, without your having to say so directly. At the same time, you could make reference to the fact that you will see her the next time you get together with the entire family. This is another indirect way of telling her that you don't want to have a separate attachment to her.
Other people prefer a slightly more direct approach in which they let the other person down easily by suggesting that they would be open to a friendship and nothing more. Such an approach might sound like, 'I think that you are a lovely person and I would like to be your friend.' If you don't even want a friendship, then you couldn't use the previous option.
Another option is to tell her a portion of your true feelings and leave out the piece that might crush her. In other words, you don't need to tell her that you aren't attracted to her, but you could tell her that, given your continued ties with her sister and the entire family, you would feel akward dating her. Let me know how you make out. works out.
I commend your level of sensitivity and consideration for others. You are certainly a fine friend and lover.