IIRC he had given the box to someone else, so it wouldn't have been sending him messages that day.
I think this question is being over-analysed anyway. The simplest explanation is the most likely. He knew the PO was investigating to find out who these girls were, so he didnt really have a choice other than to bring them in and hope for the best. The other option would be to have done nothing, and continued claiming that they were his brothers kids who were visiting for the day. But, from all accounts, the latest PO he had was much more diligent that previous , and PG would have known this, so doing nothing would have likely resulted in the PO following up with the brother and then finding out that there were no girls. That would have resulted in PG being placed under a microscope, which he most definitely would not have wanted.
Taking the girls in would then have been a pre-emptive action, since it would have taken the brother (and any interviews with him, since the girls were physically present) out of the picture. He would have been relying on the family being interviewed together, where he would have controlled the flow of the conversation, but that didnt happen.