Check with your plan administrator; more than likely, she left a beneficiary designation. If she was given the right to so designate and she...

Is my marriage legal and do I need to get a divorce in the US?

You can file in CA; the only complication is serving her in Mexico. Here’s a link to info: https://www.courts.ca.gov/selfhelp-divorce.htm

Do I need to refile for divorce if it wasn't granted the first time?

Your case from 2012 has almost certainly been dismissed for lack of any action but you can check with the courthouse where it was filed to be certain.

Why did my kids get taken for me

...probably because you weren’t paying attention which results in a finding of “failure to protect” from the negligence of the other parent.

Do I have to let my 3 year old and 5 year old spend night with their father if he's lieing about certain things?

You can address any concerns you have by filing another RFO to modify or define custodial requirements but you can’t unilaterally deny custodial time.

I don’t see how the U.S. court would have jurisdiction over a Mexican resident.

You need to file the FL-170 (Dec for Default or Uncontested)

Can my brother take me to court for temporary custody of my child... he had her for 4 years I’ve been trying to get her back 2 ?

Yes - although the appropriate request is a petition for guardianship.

My husband and I are separating and discussing divorce. How to I protect my pension from my husband and his mistress?

You cannot; he has earned a property interest as a percentage earned during marriage. You could trade it’s value with other assets if he is amenable.

Can my sister get the cops involve if I don't agree with her how I raise my kids?

Not the cops - but she could report you to CPS. Perhaps try discussing her issues with her to ease her concerns while not invalidating her...