Wow, I am sorry that your sister did this to you. I have been burned a number of times in a similar way, except it was never a close relative - only more distant relatives or close friends.

Part of me feels that you need to figure out which is more important - the money or your relationship with your sister. Could you handle "letting it go"? Would you even be able to salvage the relationship at that point?

It would be great if you could deal with this in therapy - WITH your sister would be even more ideal, but probably unrealistic.

The other part of me says that you should sue her...