How does a court determine which parent gets custody?

The court considers what is in the best interests of the child in determining custody.  There are two types of custody; physical and legal.  Physical custody means the parent with whom the child lives.  Legal custody means making decisions such as education, healthcare, etc. for the child.

Since the child is only 22 months old and is accustomed to living with her mother, it is unlikely a court would remove the child from her home and award physical custody to your brother.  It is possible that the court might award joint legal custody to both parents.

Custody is a separate issue from visitation.  Your brother has a right to visitation.  It would be advisable for your brother to file an Order to Show Cause (court form) requesting visitation along with a supporting declaration.  Your brother should also include in the Order to Show Cause and the supporting declaration his request for custody.  When an Order to Show Cause is filed with the court, the court will schedule a hearing on the issues raised in the Order to Show Cause.  The Order to Show Cause and supporting declaration stating the facts in favor of your brother's requests for custody and visitation should be filed with the court along with a proof of service.  The proof of service verifies the date of mailing to the other parent.  Your brother can use a court form proof of service or can write his own.  If he writes his own proof of service, it would just say that he is at least 18 and the attached documents were sent via first class mail unless stated otherwise to ___________ (name and address of other parent) on ___________ (date).  Your brother signs and dates the proof of service at the bottom of the document.  The date your brother signs should be the same as the date of mailing and the same date the documents are filed with the court.