I'm sorry for the refusal. Getting a visit visa really is a lottery. If ECO isn't really convinced of her ties in the Philippines, it would be a refusal. How old is your wife's niece? Does she have any other assets and reasons to go back to the Philippines other than her work in December? Sometimes, it's the duration of stay here in the UK (i.e. 2 weeks, 3 months, 6 months, etc.) can be the key too. Did you state she'll stay for 6 months? Also, what reason did you mention for her reasons to come here to the UK?