Yes I can see the point you are making but note how Violet Rose doesn't mention how much he gives her a week towards his 4 daughters but she can mention figures like €2,900 because yes that is seen as over the top, wonder how over the top the weekly maintenance payments he gives her are. It could be possible that what she is in fact looking for is his arrears of maintenance for the year and she wants it all for Christmas, so if it's due she's right to look for it.
We have to be fair to both of them as all things that are said third hand are more than likely not the real truth as I always say from personal experience, there's his version, her version and then the truth.
To get back to the original posters question, I understand that when you are receiving payments like your sisters that sometimes you retain the payment for the first year after going back to work, then it is reduced to 75% following that and so on. It also depends on her take home pay from her job. My advice would be go to her local Social Welfare Office/ Community Care Centre and ask them exactly what here losses might be should she obtain part-time employment.