34-year-old aunt refuses to look after her 6-year-old nephew while her sister is in surgery because he's constantly mean to her: 'My sister called me a selfish cow'

Children will listen to what their parents say and model their behavior after them, and this boy is no exception

Some kids are a joy to babysit, and others are an absolute nightmare. If a parent constantly has to beg everyone in their family to babysit their kids, chances are it's not because everyone in the family is heartless and doesn't want to help out. It's much more likely that the kid is not particularly pleasant to babysit, and if that's the case, there's a good chance their parents aren't a joy either. 

Good parents can raise badly behaved kids, especially when they're toddlers. Toddlers are often mean and nasty, but if a child continues to misbehave like a toddler by the time he's 6 or 7, there's a better chance that his bad behavior has something to do with the kind of parenting he's receiving at home. 

Most family members and friends would babysit a kid they didn't particularly care for if there was an actual emergency and their parents couldn't watch them. But that depends on someone's definition of "emergency." If a parent doesn't want to pay a babysitter because they're too cheap and want their family to watch their badly behaved kid for free, that is not an emergency.