Okay so I will add a little more hx:
In the ED, the attending physician when doing the initial physical finds bruise marks on Michelle's arms that are in the shape of a human handprint. After this discovery - a CPS referral is made. What should CPS do?
In this case they should take Michelle and her brother to the closest Children's Advocacy Center - assuming one is close.
There they would also contact the police and a trained interviewer from the CAC would interview Michelle in a teen friendly atmosphere. These interviewers (our Kellie for example --- kudos to you Kellie -- I know that work is hard to do) can be very good at getting children to open up and to actually tell what is happening to them. CAC interviewers aren't always successful but the more interviews they have done, the better they become.
Let's first assume Michelle opens up and tells all. Then Michelle would be asked to have a forensic exam after the interview by a physician trained in this area. Michelle however could refuse the exam and at 14, that is her choice. Her 10 year old brother would also have an interview and an exam. No matter how you slice this one, it will be traumatic but doing it at a CAC will give better results. In the past some of these interviews might begin in the police station (not very child friendly - and not very conducive to opening up or it may be done by a social worker a little better but still difficult). If Michelle refuses to discuss how she got the bruises and the broken wrist, it is possible that her 10 year old brother who would also be interviewed may reveal detail he knows, for example did he hear the argument? Did he hear arguments in the past? Any information he provided could be shared with Michelle and then she may decide that it is time to tell what happened to her. So assuming she does tell.
Then comes the hard part. Likely both children will be taken into CPS custody. The mother may be allowed to get her children back in time after she gets rid of the boyfriend. But in the immediate circumstance a determination of safe placement for both children would have to be made. Hopefully there is a trusted relative such as an aunt or uncle who could take both children. I am not sure if the state would let the children go to NY with the father but he may be able to come back to California for a while. If no safe relative placement could be found and if foster placement is the only option, the best option would be a home that can keep both siblings together and in the same school district so as not to further exacerbate trauma anymore than it has to be. If this isn't possible then I am really sad for both children.
Siblings in Foster Care Maintaining the Ties that Bind (University of Minnesota)