When someone leaves everything in their will to their spouse, without leaving anything to their children, it is because that is what they want to happen. Your issue here is with your parent, not your step-parent. Sometimes, your parent will leave everything to their spouse (your step-parent) with a vague promise from them that they will pass everything to you when the step parent dies but nothing in writing, and this is useless. Your parent believes their new spouse to be a good person who will honour this but once your parent is dead there is no way they can make sure their spouse follows their intentions.
Occasionally, as was the case with our family, the parent who dies does not leave a valid will. Our stepmother had written instructions and a good understanding of our dad’s intentions wrt his estate and how it was to be divided, but the law (intestacy) allowed her to take a far greater share than he had intended to so she did. The only reasons we can come to for why she did this is because she is a complete fucking witch. Our family has never recovered - she locked us out of our dad’s home (which was not even her home at the time) less than 24hrs after he died and we were never got a single photograph of him after he died, let alone any of the collectibles and possessions that he told us (and which were put in writing, just not correctly signed) would be ours after he was gone.
What I have found on MN is that there is a very cold-hearted approach to inheritances. When someone dies and leaves someone bereft it is often stated that you are not entitled to anything anyway, it was never yours until it was passed to you legitimately in a will. The sad fact and reality is that not everyone has a valid will, but complicated family situations mean that valid wills (where the person the will belongs to understands ALL of the implications and repercussions) are more important than ever, as are the often difficult conversations about who will be left what after someone dies. It is not unreasonable for a parent to want to leave their new spouse a home, an income after they have died, possessions that they shared and acquired together during their marriage. But it is also not unreasonable for someone to want to pass family money and possessions from before their new marriage to their children, and it is not unreasonable for those children to want to inherit from their parent. A clear conversation and a correctly written will avoids any of the sad situations where someone unexpectedly feels hard done by.
In my family’s situation, my dad had intended for his share of his family’s wealth, which he had inherited himself from his parents and grandparents, to be passed to his children, and for the portion of his possessions and money he had earned for himself in his lifetime to go to his estranged second wife. His will had been written and not correctly witnessed, so she took the lot, despite it being more money than she knows what to do with, and enough to have supported not only her family but the families of all my siblings too. I hope rots in hell for her actions and greed, but have resigned myself to the fact that she will probably just waste most of it and pass the rest on to her children who she had with another man after she had separated from my dad.
Long explanation, but if you just skipped to the end for the punchline - the reason some people don’t share when they are sole beneficiaries of an inheritance is because they might be cunts.