Sibling Rivalry and Spoiled Niece

My 11-year-old niece is the youngest, only girl, and somewhat spoiled (due to being sick

when little). When she wanted a mother-daughter birthday trip, her parents refused,

feeling excluded, and planned nothing. I took her on a trip instead. Later,

one brother requested a family trip to Mexico without my niece. Her parents agreed,

calling it fair punishment for her earlier exclusion request, and asked me to

watch her for 10 days. Upset by the punishment, I planned a surprise 10-day work trip

(Miami, Orlando, NYC) and informed her parents last-minute that I would take

her, forcing their agreement as they had no other childcare. She had an extravagant