That's a tricky one. I think I'd be inclined to believe it's a lazy way of making sure all the brothers, sisters, nieces and nephews benefit without specifically naming them and having to update the will whenever one of them died.
As a result, I'd take it to mean that the testator had many brothers and sisters, but that the statement about child or children of deceased brothers and sisters would apply at the time the will was proved, rather than when it was written.
It is somewhat open to interpretation though, especially with "now" being the 2nd last word in your quoted excerpt. Quite confusing really. I wonder if there's any correspondence regarding the proving of the will in the local archives, deposited by the solicitor? Sometimes you can be lucky and find something.