VQ I'm glad that Bags explained that to you.
The reason I asked about hours worked is that I'm trying to get a handle on this as far as gains and losses go. By my calculations your DD will be £910 better off on the new 'minimum' wage than the current one (7.20-6.50)x25 hours x 52 weeks.
I understand that as a self-employed window cleaner your SiL cannot award himself a pay rise. Perhaps with his pizza delivery job he earns more than her, perhaps less. I don't know. Both are probably below the present tax threshold or at least the new one.
So, what those of us who have never had or understood Working Tax Credits (because they didn't exist for our generation) is how much a family in this situation would have received in these benefits and others before the budget and how much after.
Only with these facts and figures can we comment on, or understand, how their finances will change. Please don't attribute any sinister motives to the question as I'm sure there are many others on this forum who, like me, haven't a clue.