Is there a good hook for on CiviCRM sign in?
[This is to display a message upon login in a cms-independent extension.]
Login is a CMS event, not a CiviCRM event. This means you can access it readily from a CMS plugin/extension/module ... but not from a CiviCRM extension.
If you wanted to access this from a CiviCRM extension, you'd need to use some event which fires at the time of login. You might be able to identify one using the
contrib.showallthehooks extension to reveal which hooks get fired at the point of login. Might need to check it was going to fire for each of the CMS platforms, since behaviour could vary.