@Juan Bustos
This is one of the great things about tax sales; they clean up title issues where people didn't do what they were supposed to. Once the property goes through the tax sale, and the relevant passage of time, the title issues will disappear. While that doesn't help you before the sale at least the property can be returned to an investable condition.
There are a couple of options depending on the amounts involved and your risk tolerance. Have one of the sisters talk to the county to see if a partial payment can postpone the sale, then get her to give you an option to buy it for the amount that will postpone the sale and ensure that gets to the county. Once the sale is postponed, you can pursue getting the title issues cleaned up and making a deal with the other sister. Yes, you are risking all of the money you put into the deal if you cant get the postponement or if you cant get the other sister to agree to sell to you. But depending on the dollars, whether you think you can get a good agreement with both sisters and if you sure there isn't another potential heir, it might be worthwhile.
It's a complicated play and I wouldn't want it to be my first foray. But with the prices these properties are bringing at the auction it might be something I would attempt.
Good luck! Let me know how it goes.