Guardianship of my Nephew

[mrtnzfamily243]

Hi, I have guardianship of my sister- in- law's three older children; but my concern is for the one she still has. I live in California, which is where the mother lived up until 3 weeks ago, when they moved to Las vegas. They stayed with a friend for two weeks. During the time they stayed with the friend, she wittnessed the mother hit the child ( who just turned one April 10th) in the face and use a plastic bag for diapers because she ran out of diapers and they spent the last of their money on cigarettes. She threatened to beat his fing but if he moved. After that happen they moved into a motel. They ran out of food on Sunday and their friend felt bad for them and bought them dinner on Wednesday. Yesterday, we had a 6 minute fever induced seizure because he had a fever of 104 for at least two days and it was left untreated. I want to know how I get at least temporary guardianship of my nephew. Do I need to go to las vegas or can I apply for it here? We are very worried something is going to happen to him.
They both abuse meth and prescription pills and the father is an alcoholic. The mother is also 8 months pregnant and smokes a pack a day.