Tax credits reduce incentive to work more?

[11stone4]

My best friend Has found this to be the case. The more hours she works the less tax credits she gets. So what's the point exactly to work longer hours to get working and child tax credits to be reduced. I'm a SAHM FWIW

I work 25.5 hrs a week and thats over 3 full days and 3 hrs on the 4th day.Am i entitled to claim social for the rest? I know the 3hrs on the 4th day was stopping me before but has its all changed?