I've never contested a will but I'm aware I may have to in due course, so I've read around the subject a little.
A will can be overturned not only because of improprieties in its making (issues around capacity or coercion, for example) but also because of its effects. Factors that may be taken into account include current and future need, and expectations of inheritance. Expectations might derive from actual promises, or from, for example, any kind of reciprocity the will was expected to express, such as in acknowledgement of care of an elderly parent by an adult child (I realise this may go against the grain with some people morally, but the point is that the law acknowledges such expectations as reasonable). Also germane might be the origin of the estate, so, for example, it might be argued that a child could expect to inherit a proportion of the estate from a parent who originally inherited from their deceased spouse (i.e. the child's other parent), presuming no complications like remarriage.
I was going to draw your attention to the Ilott case but see you're already familiar with it. Worth remembering that in that case there were no siblings or other individual beneficiaries, only charity bequests (cat homes, I think!) which made the expectations clearer to quantify.
You need expert legal advice really, not opinions from randoms on the internet, so if you have a lawyer already I suggest you listen to what they're telling you, and if you're questioning the merit of the legal opinion you've already had then consult someone new to see if you get the same advice or not. Good advice now is crucial in deciding how deep to get into this.
I'm sorry for your bereavement and the pain its consequences are causing you. 