As I read your post, the will made 15 years ago was made AFTER your uncle remarried. (That's important as marriage normally invalidates any existing will). So that will remains valid unless your uncle has revoked it in writing (or by physically destroying it himself).
So if the new will is not signed the original will determines the distribution of your uncle's estate. However his wife can contest the terms of that will, on the grounds that it fails to make 'reasonable financial provision' for her:
http://www.legislation.gov.uk/ukpga/1975/63
If the new will is signed prior to your uncle's death, both his wife and son can challenge the terms of the will on the grounds of 'reasonable financial provision'.
The step-daughter who is to receive nothing from either will could also make such a challenge (to whichever will was valid) if she could show that she was treated as if she was your uncle's daughter.
Your uncle is, of course, free to give away his money to anyone prior to his death. However such gifts will still count towards the value of the estate when determining whether there's any liability for Inheritance Tax:
http://www.hmrc.gov.u...ey-property/index.htm
Chris