It is extremely rare for an attorney to take on a criminal case pro bono. This is due in large part because people who cannot afford an attorney get one for free from the state. In your sisters case if she isn't satisfied with her attorney she should mention it to the court and ask that a new attorney be assigned to represent her.
It is unlikrly to find a pro bono criminal defense lawyer because there are public defenders. She can ask the Judge to appoint someone else.
I am a former federal and State prosecutor and now handle criminal defense and personal injury/civil rights cases. Feel free to check out my web site and contact me at (212) 577-9797 or via email at Eric@RothsteinLawNY.com. I was named to the Super Lawyers list as one of the top attorneys in New York for 2012. No more than 5 percent of the lawyers in the state are selected by Super Lawyers. The above answer is for informational purposes only and not meant as legal advice.
Keep in mind that if she asks the judge to assign her a different attorney, a) she might like the new attorney even less than her current one, and b) the new attorney will require time to get up to speed on the case, thus delaying it even more.