You probably can; it would depend on their local restrictions:
Your biggest problem is that you probably cannot bid high enough to get it;
The Bank usually bids the amount of the payoff, and that would be more than the house is worth.
A lot of people, the last few years, have tried to find ways around foreclosures and shortsales, and it really cannot be done.
Your sister owes more than the house is worth, and that money has to come from somewhere.
If you have money coming out the wazoo, you could just arrange with her bank to get her current, but that would not be a wise financial decision; it would save her home and keep her from moving, but....