Can we fight our father's Will?

If the will was executed correctly (2 witnesses, notarized, etc.), if your father was competent, and if he was not unduly influenced by someone else, then it is going to be tough.  You should speak to counsel about your chances.  Generally speaking, the courts hate to overturn wills, and will only do so if you can make a strong case that the testator was either incompetent or unduly influenced, unless the will document itself or the signing of the will was somehow faulty.    

The first thing that needs to happen is that you need to contact the executor of the estate.  This is the person that is in charge of distributing the estate to the people stated in the will.  The will shall control the disposition of the assets.  If you and your siblings are trying to obtain property/assets that are not provided for in the will, then the executor is not going to give them to you.  I also suggest that you each hire lawyers to represent you in this matter so that the lawyers can sit down and figure out a way to resolve this.  Having a lawyer involved will help facilitate a resolution of this matter.  Regardless of the promises your dad made to your mom, the will controls.

What you are asking if it's possible to contest a will or challenge it's validity if you feel it wasn't written correctly.  Just make sure that before you do, it does not have a "no contest" clause in it.

No Contest Clauses

When a will is created, a "no contest" clause can included to stop anyone from challenging your wishes. Basically, a no-contest clause forces a beneficiary to choose between taking what you give them under the will or getting nothing at all.

If a beneficiary still tries to contest the will and the no contest clause is enforced then that beneficiary will be completely disinherited and his or her share will be distributed into the residuary estate, unless there are directions regarding what to do with the share. When writing your will you can check with your state to find out if there are laws that will enforce a no contest clause.

You Must Have Standing

One of the requirements for contesting a will it that you somehow have a connection to the will and feel harmed by its contents. You either have to be named in the will, but feel that you should have inherited differently, or weren't named in the will and should have been named in it or would have received money if the person had died without a will.

Valid Reasons for Contesting a Will

You also must have a valid reason to challenge a will, such as undue influence, fraud, mistake, or loss of mental capacity of the testator. Simply being upset with your inheritance amount isn't an actual reason to contest a will.

Seek legal help in your area with this.  It is not an easy task to accomplish.  And you may be depleting or losing your inheritance in doing so.