Are there any checks and balances to make sure a Will is completed correctly?

I am so sorry for your loss.  It really may be a good idea for you to take a trip to California or to have an attorney in the area take a look at the probate of the Will for you.  Did she give you a reason for the reduction of the inheritance?  To answer your question yes, there are checks and balances, so to speak, in an estate process.  The personal representative of the estate - the executor - must file an accounting of the estate, meaning that they must present to the court the financial accounting of the assets and debts and distribution.  But you as a beneficiary have the right to make sure that everything is on the up and up and to question the distribution and the executor if you think something is awry.  Seek legal help.  Good luck.   

You can bring a legal action. It's not up to your step mother--the will controls the distribution of your father's assets (with a few exceptions). You have to get what's been left to you in the will; certainly, your step mother can ask if you'll disclaim some of it because she needs the money or assets, but it's up to you to decide to do that or not. If your step mother won't voluntarily honor the terms of the will, you'll need to go to court, so you should consult with an attorney.

Note that certain assets do not go through the will: for example, if  your father and step mother jointly owned a house or a bank account, together, that asset automatically goes to her. Life insurance goes to whomever the designated beneficiary is. There are a few other exceptions, which your attorney can discuss with you--the rule though is, except for the exceptions, the assets will be distributed according to the will.