How does my Sister in law get custody of her grandson

There is only one choice in the absence of a pending paternity or other proceeding between the niece and the father: A guardianship proceeding. The niece's mother has no standing to file any other type of action; however, if another action is pending (again, a dissolution or a paternity action between the bio-parents, for instance) your sister can seek custody pursuant to Family Code section 3041, which deals with custody to nonparents, by applying to be joined into those proceedings. Your sister in that case would be presumed to be the more appropriate custodian of the boy IF she can establish a stable placement exists with her and that she has "assumed, on a day-to-day basis, the role of his ... parent, fulfilling both the child's physical needs and the child's psychological needs for care and affection, and who has assumed that role for a substantial period of time." The last portion is critical. If the child's mother gets her act together, or pretends to, its a bit close whether a one year period will satisfy a judge (has the situation been the same since birth?). Also, I would want to know whether the child's mother live's in your sister's house or has just parked the child there. If the former, it will be tougher. The greatest danger here is that the child's mother will move to a bad housing situation just to defeat her mother. If your sister is in control now and the child is protected, she should consult an attorney immediately in her area so that she makes all the right moves until the inevitable time comes for her to take action.