As I interpret your secondary post, you're not seeking to show that the will has no validity (because, for example, the testator was coerced into writing it or because it wasn't properly witnessed). So you must be seeking to show that the will fails to make "such financial provision as it would be reasonable in all the circumstances of the case for [you] to receive for [your] maintenance"
https://www.legislation.gov.uk/ukpga/1975/63
So you'd need to show
why your stepmother had a duty to maintain you. You can't simply say "She was my Mum, so therefore I'm entitled to something (or to more than I've already got from the will)".
A parent (or step-parent) is never obliged to leave a single penny to their children (or step-children) and they're perfectly entitled to write a will which totally excludes their offspring (or, indeed, any other members of their family) from receiving anything. The provisions of that will MUST stand
unless the child (or stepchild) can show that they had a reasonable right to expect the testator to maintain them.
For example, if a student was half-way through university at the time of their parent's death, and the parent had been funding those studies (and would obviously have continued to do so if they'd not died), then if the will left nothing to the student they would have a good chance of getting a court to alter the distribution of the parent's estate.
However if, when Joe Bloggs dies, his daughter is happily living her own life with her husband (and in no way dependent upon Joe) then, if the will leaves nothing to her, she has absolutely no right whatsoever to get the terms of the will varied.
Many people (including me) write wills that don't leave anything to their family members. A court will NEVER vary the terms of any such will UNLESS the application comes from a spouse or civil partner of the testator (irrespective of whether they need the money for their maintenance) OR, where the application comes from another person (such as you), it can be shown that the applicant had a reasonable right to expect the testator to MAINTAIN them.