Why did they reverse the decision?
Rumour has it that after the cross-sibling rule was removed, they were concerned that the intake had become less academically able, which would affect results and hence league table position. They are now academies, which gives them more autonomy over their admissions criteria.
Is it likely that they will remove it again?
No. Especially not for this coming year (2014 intake).
They reversed it because they never wanted to see it go in the first place. It was only removed when a group of parents complained to the adjudicator and the adjudicator forced them to remove the policy. I believe the school has always wanted to provide the benefits of sibling places like all other (non fully selective) school do. As they are single sex, it made sense to partner with each other to provide the cross sibling places.
Whilst there might be some evidence that children from the same family group are likely to have similar academic ability it is probably more aimed at attracting parents who might otherwise discount a single sex school if they have DD and DS. When my DD was initially viewing schools her favourite was WGGS but I favoured Parmiters, not because it was better but as it would give an automatic place to my DS without him having to take the consortium test. Not saying I am correct, just saying what I thought at the time.
The grammar schools are very popular and even when there was no sibling policy you had to live very close to the school (700metres) and despite it being our nearest school we had no chance on distance so had to rely on academic or music as our entry method and if there was a way to avoid doing it twice with all the angst and cost that entails I would have taken it.
Ultimately, DD got into WGGS so we had to do do it twice anyway but I personally feel it would be more consistent with all the other local schools which allow siblings to attend if the cross sibling policy continues. I also appreciate that others may not agree.