Is preparing eggs for a meal considered as sinful in Buddhism?
I think the idea of sinful or not doesn't apply to Buddhism. There are only wholesome/skillful actions and unwholesome/unskillful actions.  Karma = intention.  

As long as you are mindful of what you're doing and your intention is to prepare a meal for nourishment, instead of causing harm in any way, cracking an egg during the meal preparation doesn't seem to be a cause for bad karma.