Just because you are not divorced she cannot automatically challenge you Will.
As far as it possible the courts will try to put into effect the intentions of the Will maker rather than interfere with them.
They will interfere where they believe that the deceased would have wanted to leave something to somebody but did not get round to changing their Will. A good example is the grandparent supporting a grandchild at college at the time when they die. A court would presume that the grandparent would have wanted to continue to do so.
Your siblings would not succeed as against you son it is natural to go down the family line.
Anyone can witness your Will apart from the beneficiary , your eldest son.
My name is ***** ***** I have been a solicitor for more than 30 years.
I am afraid that matters are not quite so straightforward so far as your wife is concerned.
If there has been no previous division of the matrimonial assets then it is likely that she will potentially be able to make a claim using the Inheritance (Provision for Family and Dependants ) Act 1975
Claire is correct that if you did not reach a financial settlement on separation that there is a potential claim. If your financial position was and is much better than hers after separation she may be successful even after the passage of time as such a statement to accompany the Will can explain why you have not left her anything. The statement is persuasive but would not prevent a court from giving her something if they felt it was the right thing to do.