My sister-in-law is a horrible person, and I was wondering what would happen if my brother passed away after my parents' will was already executed. In the event my brother passed first, it's obvious I would be the sole heir. However, if my parents pass first and their will was distributed to me and my brother and then my brother passed after them, can my parents add a clause to their will to deny his wife any part of the estate? If he had children, I'm sure they'd want it to go to them. However, if he didn't have children, can a clause be added to have it go back to me? She's entitled to 1/2 their accumulated assets during the marriage, but is there any way to deny her anything related to my parents' will?
They might need to do this via a trust rather than a bequest.
Someone I know has a trust from their parents that says they can use the money during their life for their needs (and lists a fairly comprehensive set of examples) and upon their death the remainder goes to their children. I think you can has some control over what happens to your money after you die.
I think your parents could leave your brother's portion to him via a trust that provided for him to use the money during his life but had the remainder go to his children. As someone said in answer to that other question, if money is left to your brother in their will, it is his at that point and he can leave it to his wife or to his dog if he likes.
Their attorney may be able to structure a trust that solves the problem.
In some jurisdictions, certain relatives cannot be disinherited without their consent (e.g. in community property states, your wife's right of inheritance is independent of any will). Daughter-in-law is not one of the statutorily-protected relations. So, in will, a parent can distributed their property as they see fit -- all to sister 1, split evenly between the brothers, split evenly between oldest son and oldest daughter, and so on. They can explicitly include a daughter-in-law if they want; they can include me if they want. Nobody gets a share if they are not mentioned.
A will can have conditions, e.g. "all to my wife, but if she dies before me, all split evenly among children A,B,C,D". The hard part if for the testator to decide on / think about a hierarchy, such as "Wife first; if wife dead, oldest female child; if oldest female child dead then children of oldest female child...". Painful statements like "Person X shall in no circumstance receive any portion of the estate" indicates testator's intent, and following the testator's wishes (hence "will") is the basic principle of inheritance law.
It's certainly possible to write a will, especially if it is really simple. Attorney's generally do not charge a huge sum to write this sort of stuff up. What you really need is to figure out in advance is what you want the will to do.