Things that are clearly not illegal:

*Cutting herself
*Screaming at him
*Making allegations about child abuse, in the absence of clear information to the contrary
*Using the baby as a way to control knowing where your nephew is
*Threatening to kill two of his friends if they go near him
*Fooling the judge into thinking anything (perjury is one thing, "fooling" a judge is something else entirely)
*Driving by your nephew's house (this in and of itself is not stalking under California law)
*Walking back and forth in front of his garage, in the absence of criteria needed to establish trespass
*Getting a restraining order against anyone
*Her sister's behavior as it relates to anything this girl is doing

Things that are illegal according to your descriptions:

*Battery on your nephew
*Vandalism to the windshield
*Taking property out of your sister's truck

Some of the other things might be illegal, but there's not enough information to determine based on what you've written here.

The crimes you did list are misdemeanors (battery on your nephew may be a felony depending on the circumstances but my guess is it would be treated as a misdemeanor even if it was a technical felony). Misdemeanors are punishable in California by up to one year in jail. They are also punishable by much less than that.

What you and I consider to be an unfit mother is not the same as what the law considers to be an unfit mother. If she treats the child acceptably (you didn't indicate anything to the contrary), then she very likely would be a fit mother under the law. By it's very definition, a fit or unfit mother has to do with the relationship between the mother and the child…not the mother and all the other adults around her.

Slander is not a criminal act, it's a civil one. She can be sued for slander if the violations can be proven.

What do I think his chances are in court? Despite the vast amount of information you've provided, it's not possible for anyone here to tell you. Court isn't about "chances," it's about what can be proven. There are always at least two sides to every story and there rules of evidence which dictate what kind of information can and cannot be considered in court.

Your nephew needs to obtain competent counsel.