how couldnt she see it was not her son

Personally, I think she knew, but given that the only people whom could help her were pushing the fake son onto her. I think she kind of capitulated for the moment, trying to figure out how to get them to continue to help her while humoring them in the short-term. Can't exactly recall the time line here, but wasn't this about the time that she enlisted the radio preacher to help her figure out where her boy was, and why the Police were perpretrating this hoax on her? Remember, at that time women had virtually no rights and were more 'subservient' in our society. The police had her institutionalized for disagreeing with them. Knowing all of those kinds of things about how the system worked against women speaking out, might cause someone to 'take a fake child' for the short-term, until they could figure out another strategy.

Women in those days were forced into being passive dunces who were not allowed to have thoughts of their own but to have men in authority tell them what to think.

She was bullied into taking the boy and smiling for the cameras so that the LAPD didn't appear incompetent.

The first thing out of her mouth when she saw the boy at the train station was "That's not my son." She knew it wasn't her son, but since the cops were so corrupt and on the brink of being completely outed as such, they forced her to take the boy home, along with photographic evidence to make them look competent.

She knew right away the boy wasn't her son. She was bullied into taking the boy against her better judgement. She just wanted to believe that it was possible that the police were right and that she was just in shock, but when she saw him in the tub she knew it was impossible that he was.

I don't know shat you were watching but the first thing she said when she saw that kid was "That's not my son" and then the police detective says what? Yes it is. And keeps pushing it until he turns looks at all the cameras and then says why don't you take him home on a trial basis to see if it later comes back to you once he's in familiar environment. Before that even happens he says the department doesn't usually get good press and that's how he becomes adamant.

I can't decide if this movie is a statement on how corrupt the justice system was back then or how women were pressured into accepting decisions by authority figures. The whole premise was just so frustratingly ridiculous.

I dont know if someone already answered similar down the thread... but it was explained during the movie

Christine Collins: Why would they do this?
Rev. Gustav Briegleb: To avoid admitting they made a mistake when they brought back the wrong boy. Of course, anyone reading the newspaper with half a brain would see through it instantly. Sadly, that would exclude about half the readership of the Times. Mrs. Collins, I have made it my mission in life to bring to light all the things the LAPD wish none of us ever knew about. A department ruled by violence, abuse, murder, corruption and intimidation. When Chief Davis took over the force two years ago, he said...
Chief James E. Davis: We will hold court against gunmen in the streets of Los Angeles. I want them brought in dead, not alive, and I will reprimand any officer who shows the least mercy to a criminal.
Rev. Gustav Briegleb: He picked fifty of the most violent cops on the force, gave them machine guns and permission to shoot anyone who got in their way. He called them the Gun Squad. No lawyers, no trials, no questions, no suspensions, no investigations, just piles of bodies. Bodies in the morgues, bodies in the hospitals, bodies by the side of the road, and not because the LAPD wanted to wipe out crime. No. The LAPD wanted to wipe out the competition. Mayor Cryer and half the force are on the take: gambling, prostitution, bootlegging, you name it. Because once you give people the freedom to do whatever they want, as the Lord found in the Garden of Eden, they will do exactly that. This police department does not tolerate dissent or contradiction or even embarrassment. And you are in a position to embarrass them and they do not like it. They will do anything in their power to discredit you. I've seen it happen too many times to start going blind now. That's why I wanted to meet you, to let you know what you're getting yourself into and to help you fight it, if you choose to.
Christine Collins: Reverend, I appreciate everything that you're doing and everything that you said, but I'm not on a mission. I just want my son home.

:)