I mean, the technical buying and selling is easy but knowing what to buy and sell and how to time it isn’t obvious. Automatically buying low cost index funds is super easy and generally yields the best outcome for most consumer investors. Managing a balanced portfolio of B corps and the like without taking on too much risk and ending up broke is not trivial.
Also, dividends don’t change the fact that buying stock isn’t investing in a business. Buying stocks is giving the previous owner of the stock some money and maintaining or increasing the value of the stock which impacts executive compensation.
No one is entitled to anyone’s inheritance. The ethics of the situation really depend on the details. Did one child look after the parents in their old age? Doe one child have more needs? Was there a promise to distribute everything evenly?
If the only reason for exclusion is because one child has depression or anxiety and isn’t the smartest, then that sounds pretty ableist and shitty. If the person really can’t manage the money, why not set up a trust designed to help them out without just handing over lump sums of cash? The one case where exclusion makes sense is if they require long term in patient care since at least in the US, all your money is eaten up by the medical bills before you default to Medicare (unless you have a stupid amount of money and can pay out of pocket for premium care forever)