Can I fight a will after my sister had it changed and left three children of my mothers out .

I'm not sure from what you wrote whose will was changed.  If your mother hcanged her own will, she has the right to do that.  If you are saying your sister changed your mother's will, that's a different story.  You would have to prove what your sister did, that she unduly influenced your mother, or forced her to change the will.  But none of this can be done until your mother passes away.  If she has already, you can file a challenge againt the will.