I had a somehat similiar situation, so my knowledge is based on my case from 8 years go. Don't pay attention to the woman who said that your niece has to go to CA. The baby was born in IL and they've been living in IL for more than 6 months, so they are in IL's jurisdiction. Any type of custody hearing would need to take place in IL. The father would have to file in IL. By doing so in CA, he has no grounds to stand on because CA has nothing to do with the case. I know because my son was born in CA and when he was 21 months old, I left my first husband and moved home to IL. I established residency in IL, which takes six months, then filed for divorce. Once I had IL residency, my son did, too, because he was with me. If my then-husband wanted to take any legal action, he would have to do it in IL with a lawyer also from IL.
Your niece should contact a lawyer who specializes in interstate custody law, but I'm sure the lawyer would simply have to send a letter stating that she has IL residency and that the case is under IL jurisdiction.
I know this must be so hard for your family, but rest assured that this guy has no case. He's probably just doing it to shake things up. One letter from a lawyer here should suffice. Good luck!