In both these situations, you would have a strong case if you filed a grievance against your boss. It would be much more difficult to win this kind of grievance if the contract clearly prohibited the activity. Here, the contract was clear. It specifically said you could only use sick days for your own illnesses. So, it would be hard to win a past practice grievance.
Cases involving employee benefits or privileges make for strong past practice grievances.
If workers wait years to file a grievance against a new management policy that isn't directly addressed in the contract, management may argue that it has become a past practice. This is why it is very important to challenge management actions right away when you think they may violate your rights. Many situations aren't cut and dried, and this area of labor law can be very complex. But this should not stop you from discussing the problem with coworkers and seeking advice from your steward or local union officials if you think your employer has violated your rights by changing the rules in the middle of the game.