Gaining Custody As Non-Parent Guardian

Location: Texas I've been co-parenting my little sister since she was born, and at 18 I became the primary caregiver physically and financially. Her father is absent but is supposed to pay child support. He is also an active drug user and terminally sick to my knowledge. My mother has recently gotten married and their marriage is rocky to put it kindly. If I were to move out (my mother lives with me/us I pay the bills and name is on the lease) I do believe she would fight me for custody. My sister (13) has said if it came down to it she would want to live with me, and I know that helps but isn't the end all be all. As a non-parent, what are my chances of gaining custody and what can I do to increase the odds? I have witnesses (familial and not) that can attest to my being the primary caregiver.