The CPS have to decide if there is a realistic chance of conviction. The medical evidence is clear that Falaq had a condition that could have caused the bleed into her brain at any time. Whilst this could have been triggered by physical trauma, it could also have happened without any trauma. Indeed, if the diagnosis of AVM is correct, there is only anecdotal evidence that physical trauma can cause the blood vessels involved to rupture, not scientific proof.
The prosecution needs to prove its case beyond reasonable doubt. They therefore only have a realistic chance of getting a conviction if the medical evidence is clear that something the brother did caused the bleed. Faced with medical evidence that the brother might have caused it, but it is at least equally likely that her death is totally unconnected to anything he did, there is no realistic chance of conviction. The CPS is therefore correct - he should not be charged.
If you think everyone should be charged when there is some evidence even when there is no realistic chance of conviction, the courts would be even more clogged up than they already are.