I see this happen all the time. A parent of adult children will spend much more time, money, and energy with one of their children - 90% of the time it's the child that is not doing well as the others.
Example - Coworker - ** son is educated, married has good job, nice house, kids, she does very little for him and his family ** daughter, got prego young age, is now going to school, lives in apt, she's always babysitting, and buying her granddaughter little this and that
Example - Friend, has 3 kids - ** middle child, had a child, got married, had another, live with her - she always babysits, cancels her plans to help out her daughter, buys the grandkids stuff, pays for food ** oldest, divorced, no kids, had ok job, takes a class now and then - she rarely sees her
Maybe it's because the parents feel guilty or that they've let their child down in some way and want to make it up to them. The co-worker actually said "she needs more help".
I know you're hurt and your feelings are justified. However, in your own way your going to need to learn how to deal with this situation, because chances are things aren't going to change.