If you are a biological parent of the child, then your rights trump those of her family members.  However, to make everything cleaner and assure the continuing stability of the children, you need to file a custody suit and request that the court grant you temporary custody of the child while the suit is pending.  If you can get this filed while she is still in jail, it will make it much easier for her to be served with notice.  Her incarceration will also be a factor for the court to take into consideration when resolving custody issues.