I'm 21 and my sister is 15. She desperately needs to get out of my mother's house and I need some help. How do I go about getting custody of her and what information would be pertinent to present to a judge in support of my decision

You need to provide a lot more information for us to be able to give you the best answers available. Where is her father? What does he have to say about it? What is the current custody order? Have your mother's difficulties been brought to the attention of police or social services? Are they bad enough that they would consider removal?

If there is no documented history of abuse or neglect, you will have a very hard time removing your sister against your mother's wishes. If it isn't bad enough for the state to remove her, it is unlikely they they will remove her because you think that they should. ANY involuntary removal will involve social services and a court.

However if your mother is willing to let your sister live with you, you may want to consider kinship care, which can be done informally and does not require a custody order. This (http://dhr.maryland.gov/ssa/kinship/) link has some great information on kinship care in MD.