Is it permissible to buy special things for the deceased brother's daughters without including his sons, or do they have a right to what is bought for the family?

You are not required to equalize the expenditure between your children and your brother's children, and there is no harm in preferring your own children. What is obligatory upon you, if the expenditure for your brother's children is incumbent upon you, is to spend on them what is sufficient for them. Anything beyond sufficiency is a bounty for which you will be rewarded.