So I have an exam on 18th Dec, and my niece's birthday party to attend on 13 Dec.

If I go to the birthday party, I will feel bad for not spending time on revising on my exams. If I don't go, I will feel bad for not spending time with my relatives. If I go and leave @ 30 mins, I will feel bad for spending little time there.

The alternative??? It sounds like I could leave early, and spend my Christmas with my sister!