Many people know that, on marriage, your will is automatically revoked. There is a good public policy reason for this: your spouse should not be disinherited simply because you forgot to update your will. (I touched on the issue of predatory marriages a while back.)
What many people don’t understand, however, is that separation and divorce do not automatically revoke a will. If you are divorced, your estate is treated as if your spouse died before you; he or she will not be able to be your executor or inherit, but you might want to rethink your ex-brother-in-law as your alternate executor and beneficiary from the will you did just after you got married 20 years ago.
If you are separated, it is much murkier. If you have a formal separation agreement in place, your former spouse likely won’t be able to inherit, though you still have the same problem of your alternates. If you don’t have a separation agreement, though, there are no bans on your former spouse inheriting your entire estate.
This is likely not the solution you want. If you separate, you should have your will redone as soon as possible.