Pro bono means "for the public good" so a private custody matter between battling parents does not qualify. Neither does a divorce case between two that cannot work things out. An hourly legal fee arrangment is more likely.
Many of us laywers do pro bono work to assist the homeless, aid local parks, represent issues of public concern etc.... A private litigation case does not benefit the public good.
Use the Find a Lawyer tab above to commence your search for affordable attorneys. Or, contact Legal Aid for a bit of free legal advice.
You may be able to find a lawyer to handle this matter on a pro bono basis, but another option is to see if your sister can hire a lawyer and have that lawyer request "suit money." Basically, Maryland law recognizes that both parties to a divorce should have equal access to legal representation and therefore allows the Court to award one party the costs of "reasonable and necessary" attorneys fees and other expenses (commonly known as "suit money") where the financial resources of the parties are unequal. It sounds like that very well may apply to your sister's situation. Please invite her to email me at email@example.com -- I would be happy to offer her a free consultation on this matter.