How expensive & hard is it to contest a will?

My mother was married to my dad for 35 years (all of his working career) had 2 children together (now adults) and is still married to him, but left him 15 years ago. He was verbally and physically abusive and had affairs, one of the women moved in after we left and remained with him for his last 15 years, he died of cancer. In this time he changed his will to leave everything to this woman- and she wants it all - we are not financially in a position to contend the will if it is going to cost thousands but feel why should she get everything (we are not after it all by any means) can anyone let us know if we are able to do anything. Thank you in anticipation.