There's still lots of things unknown about this.
Did she kill him in the flat then drive to her sisters?
Where were the other kids if so?
Or did she drive him to her sisters and kill him there?
Again, where were the other kids?
He'd not been in nursery since before xmas yet only drove to her sisters a week before saying he'd gone missing.
What had she told her kids?
She must have had keys to her sisters house from when she lived there and I assume her sister was out as she has not been charged with anything.
When he was "missing" there was a chap on tv saying that police still had to look at cctv, apparently there was a camera outside each apartment - so id have thought they'd have checked that first - or had she damaged it to prevent anyone knowing she was the last seen with him - or was moving him.
I usually see these stories very naively thinking that she couldn't have done that unless it was an accident and just panicked and hid his body.
Sadly a lot turn out to be very evil and terrible acts of disturbed minds.